PATENT DOCUMENT

Publication Number: US-10423265-B2
Application Number: US-201715607291-A
Country: US
Kind Code: B2

Title: Temperature compensating force sensor

Abstract:
An optical force sensor, which may be used as input to an electronic device. The optical force sensor may be configured to compensate for variations in temperature using two or more force-sensitive components that are formed from materials having different temperature- and strain-dependent responses.

Claims:
We claim: 
     
       1. A force sensor for detecting an input force on a surface of a device, the force sensor comprising:
 a substrate below the surface and comprising:
 a first strain-sensitive component having a first temperature-dependent response and disposed on a first surface of the substrate; and 
 a second strain-sensitive component having a second temperature-dependent response and disposed on a second surface of the substrate opposite the first surface; and 
 
 a sensor circuit configured to detect a change in an electrical property of the first and second strain-sensitive components, thereby generating a force input estimate that compensates for a variation in temperature between the first strain-sensitive component and second strain-sensitive component as a result of a difference between the first temperature-dependent response and the second temperature-dependent response; wherein 
 the first and second temperature-dependent responses are different. 
 
     
     
       2. The force sensor of  claim 1 , wherein
 the first strain-sensitive component has a first strain-dependent response and the second strain-sensitive component has a second strain-dependent response, and 
 a strain ratio between the first and second strain-dependent responses is different than a temperature ratio between the first and second temperature-dependent responses. 
 
     
     
       3. The force sensor of  claim 1 , wherein the substrate is positioned below a display. 
     
     
       4. The force sensor of  claim 3 , wherein the display comprises one of a light emitting diode display, a liquid crystal display, or an organic light emitting diode display. 
     
     
       5. The force sensor of  claim 1 , wherein the substrate is formed from an optically transparent material. 
     
     
       6. The force sensor of  claim 1 , wherein the substrate is disposed below a touch sensing layer positioned below the surface. 
     
     
       7. The force sensor of  claim 1 , wherein the first strain-sensitive component is formed from an optically transparent material. 
     
     
       8. The force sensor of  claim 1 , wherein the substrate comprises an array of strain-sensitive components including the first strain-sensitive component. 
     
     
       9. The force sensor of  claim 8 , wherein:
 the array of strain sensitive-components is a first array of strain-sensitive components; and 
 the substrate comprises a second array of strain-sensitive components including the second strain-sensitive component, the second array of strain-sensitive components oriented orthogonal to the first array of strain-sensitive components. 
 
     
     
       10. The force sensor of  claim 9 , wherein:
 the substrate comprises:
 a first set of electrodes disposed on the first surface and coupled to the first array of strain-sensitive components; and 
 a second set of electrodes disposed on the second surface and coupled to the second array of strain-sensitive components. 
 
 
     
     
       11. The force sensor of  claim 10 , wherein at least one electrode of the first set of electrodes is shared by at least two strain-sensitive component of the first array of strain-sensitive components. 
     
     
       12. The force sensor of  claim 1 , wherein the electrical property comprises inductance. 
     
     
       13. The force sensor of  claim 1 , wherein the electrical property comprises resistance. 
     
     
       14. A force sensor for measuring input force applied to a surface, the force sensor comprising:
 a substrate comprising:
 a primary sensing component disposed on the substrate; and 
 a reference sensing component having disposed on the substrate and offset from the primary sensing component; and 
 
 a sensor circuit configured to detect a change in an electrical property of the primary and reference sensing components, thereby generating an input force estimate that compensates for temperature of the primary sensing component as a result of a difference in location between the primary component and the reference sensing component; wherein 
 the change in the electrical property of the primary reference sensing component is different than the change in the electrical property of the secondary reference sensing component. 
 
     
     
       15. The force sensor of  claim 14 , wherein the primary sensing component is optically transparent. 
     
     
       16. The force sensor of  claim 15 , wherein the primary sensing component is disposed above a display. 
     
     
       17. The force sensor of  claim 14 , wherein:
 the reference sensing component is a first reference sensing component; and 
 the substrate further comprises an array of reference sensing components, including the reference sensing component. 
 
     
     
       18. The force sensor of  claim 17 , wherein the sensor circuit is configured to detect the change in the electrical property of the primary sensing component and each of the reference sensing components of the array of reference sensing components to generate the input force estimate. 
     
     
       19. An electronic device comprising:
 an input surface; 
 a force sensor disposed below the input surface and comprising:
 a substrate; comprising 
 an array of strain-sensitive components, the array comprising:
 a primary strain-sensitive component; and 
 a subarray of secondary strain-sensitive components; and 
 
 
 a sensor circuit configured to:
 select the primary strain-sensitive component from the array of strain-sensitive components; 
 select the subarray of secondary strain-sensitive components from the array of strain-sensitive components; and 
 detect a first change in an electrical property of the primary strain-sensitive component; and 
 detect a second change in an electrical property of the subarray of secondary selected strain-sensitive components, thereby generating an input force estimate that compensates for temperature of the selected primary sensing component as a result of a difference in location between the primary component and each secondary strain-sensitive component of the subarray of secondary strain-sensitive components; wherein 
 
 the first change is different than the second change. 
 
     
     
       20. The electronic device of  claim 19 , wherein a display is disposed between the input surface and the force sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/729,172, filed Jun. 3, 2015, and titled “Temperature Compensating Transparent Force Sensor,” which is a continuation of U.S. patent application Ser. No. 14/594,779, filed Jan. 12, 2015, and titled “Temperature Compensating Transparent Force Sensor,” which claims priority to U.S. Provisional Patent Application No. 61/926,905, filed Jan. 13, 2014, and titled “Force Sensor Using a Transparent Force-Sensitive Film,” U.S. Provisional Patent Application No. 61/937,465, filed Feb. 7, 2014, and titled “Temperature Compensating Transparent Force Sensor,” U.S. Provisional Patent Application No. 61/939,257, filed Feb. 12, 2014, and titled “Temperature Compensating Transparent Force Sensor,” U.S. Provisional Patent Application No. 61/942,021, filed Feb. 19, 2014, and titled “Multi-Layer Temperature Compensating Transparent Force Sensor,” and U.S. Provisional Patent Application No. 62/024,566, filed Jul. 15, 2014, and titled “Strain-Based Transparent Force Sensor,” the disclosure of each of which is incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This application generally relates to force sensing and more specifically to force sensing using a transparent force-sensitive component that is integrated with a display of an electronic device. 
     BACKGROUND 
     Many electronic devices include some type of user input device, including, for example, buttons, slides, scroll wheels, and similar devices or user-input elements. Some devices may include a touch sensor that is integrated or incorporated with a display screen. The touch sensor may allow a user to interact directly with user-interface elements that are presented on the display screen. However, some traditional touch sensors may only provide a location of a touch on the device. Other than location of the touch, many traditional touch sensors produce an output that is binary in nature. That is, the touch is present or it is not. 
     In some cases, it may be advantageous to detect and measure the force of a touch that is applied to a surface to provide non-binary touch input. However, there may be several challenges associated with implementing a force sensor in an electronic device. For example, temperature fluctuations in the device or environment may introduce an unacceptable amount of variability in the force measurements. Additionally, if the force sensor is incorporated with a display or transparent medium, it may be challenging to achieve both sensing performance and optical performance in a compact form factor. 
     SUMMARY 
     Embodiments described herein may relate to, include, or take the form of an optically transparent force sensor, which may be used as input to an electronic device. The optically transparent force sensor may be configured to compensate for variations in temperature using two or more force-sensitive components that are formed from materials having different temperature- and strain-dependent responses. 
     In some example embodiments, a transparent force sensor is configured to detect a force on a surface of a device. The transparent force sensor may include a first transparent force-sensitive component having a first temperature-dependent response, and a second transparent force-sensitive component having a second temperature-dependent response. A sensor circuit may be operatively coupled to the first and second transparent force-sensitive components. The sensor circuit may be configured to detect a change in an electrical property of the first and second transparent force-sensitive components and output a force estimate that compensates for a variation in temperature using the first and second temperature-dependent responses of the first and second transparent force-sensitive components. 
     In some embodiments, the first transparent force-sensitive component may have a first strain-dependent response and the second transparent force-sensitive component has a second strain-dependent response, and the a strain ratio between the first and second strain-dependent responses is different than a temperature ratio between the first and second temperature-dependent responses. In some embodiments, the sensor circuit is configured to compensate for the variation in temperature using the first strain-dependent response and the first temperature-dependent response of the first transparent force-sensitive component and the second strain-dependent response and the second temperature-based response of the second transparent force-sensitive component. In some cases, the first strain-dependent response is represented by a first strain relationship that is a linear relationship between the strain and the resistance of the first transparent force-sensitive component. In some cases, the second strain-dependent response is represented by a second strain relationship that is a linear relationship between the strain and the resistance of the second transparent force-sensitive component. Similarly, the first temperature-dependent response may represented by a first temperature relationship that is a linear relationship between the temperature and the resistance of the first transparent force-sensitive component, and the second temperature-dependent response may be represented by a second temperature relationship that is a linear relationship between the temperature and the resistance of the second transparent force-sensitive component. 
     In some embodiments, the surface of the device is a cover disposed over a display of the device, and the force estimate corresponds to the force of a touch on the surface of the cover. In some embodiments, the first transparent force-sensitive component is formed from an indium tin oxide (ITO) material and the second transparent force-sensitive component is formed from a polyethyleneioxythiophene (PEDOT) material. In some cases, the first transparent force-sensitive component is formed from a first type of PEDOT material having the first temperature-dependent response and the second transparent force-sensitive component is formed from a second type of PEDOT material having the second temperature-dependent response. In some cases, the first and second temperature-dependent responses are different from each other. 
     Some example embodiments are directed to an electronic device having a transparent force sensor. The device may include an enclosure, a display disposed within the enclosure, and a cover disposed above the display and forming a portion of an outer surface or the device. The device may also include a first array of transparent force-sensitive components disposed below the cover and having a first temperature-dependent response. A second array of transparent force-sensitive components may be disposed below the first array and have a second temperature-dependent response. The device may also include a sensor circuit that is operatively coupled to the first and second array, configured to detect a change in an electrical property of components the first and second arrays, and output a force estimate that compensates for a variation in temperature using the first and second temperature-dependent responses of the first and second transparent force-sensitive components. In some implementations, the display is disposed between the cover and the first array of transparent force-sensitive components. In some implementations, the display is disposed below the first array of transparent force-sensitive components. In some implementations, a pressure-sensitive adhesive layer may be disposed between the first array and the cover, wherein the pressure-sensitive adhesive layer has an elasticity of approximately 1 MPa. 
     In some embodiments, the first array of transparent force-sensitive components has a first strain-dependent response and the second array of transparent force-sensitive components has a second strain-dependent response, and the a strain ratio between the first and second strain-dependent responses is different than a temperature ratio between the first and second temperature-dependent responses. 
     Some example embodiments include a third array of transparent force-sensitive components that may be disposed between the first array and the second array and have the first temperature-dependent response. Some example embodiments include a fourth array of transparent force-sensitive components disposed between the third array and the second array and having the second temperature-dependent response. 
     Some example embodiments are directed to an electronic device having a transparent force sensor. The device may include an enclosure, a display disposed within the enclosure and a cover disposed above the display and forming a portion of an outer surface or the device. The device may also include a first array of transparent force-sensitive components disposed below the cover and including components having a first and second temperature-dependent response. A second array of transparent force-sensitive components may be disposed below the first array and include components having a first and second temperature-dependent response. A sensor circuit may be operatively coupled to the first and second array, configured to detect a change in an electrical property of components, and output a force estimate that compensates for a variation in temperature using the first and second temperature-dependent responses. 
     In some example embodiments, the first array of transparent force-components includes alternating structures formed from an indium tin oxide material and a PEDOT material. In some embodiments, the first array of transparent force-components includes alternating structures formed from a first type of PEDOT material having the first temperature-dependent response and a second type of PEDOT material having the second temperature-dependent response. 
     Some example embodiments are directed to a transparent force sensor for detecting a force on a surface of a device. The transparent force sensor may include a transparent force-sensitive component including an array of pixel elements. The array of pixel elements may include a primary pixel and at least one reference pixel. The at least on reference pixel may be separated by the primary pixel by at least one other pixel element. A sensor circuit may be configured to form an electrical bridge circuit across the primary pixel and the at least one reference pixel to detect a change in resistance of the primary pixel. In some embodiments, the at least one reference pixel and the primary pixel are subjected to similar thermal conditions and are subjected to different strain conditions when the force is applied to the surface of the device. In some cases, the at least one reference pixel is formed on a different layer than the primary pixel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made to representative embodiments illustrated in the accompanying figures. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the described embodiments as defined by the appended claims. 
         FIG. 1  depicts an example electronic device. 
         FIG. 2A  depicts a top view of an example of an force-sensitive structure including two force-sensitive components. 
         FIG. 2B  depicts a side view of an example of an force-sensitive structure including two force-sensitive components. 
         FIG. 3A  depicts a top view of another example of a force-sensitive structure including multiple force-sensitive components. 
         FIG. 3B  depicts a side view of another example of a force-sensitive structure including multiple force-sensitive components. 
         FIG. 4  depicts another example of a force-sensitive structure including a force-sensitive component and associated sense circuitry. 
         FIG. 5  depicts another example of a force-sensitive structure including a force-sensitive component. 
         FIGS. 6 and 7  depict example configurations of force-sensitive structures having three force-sensitive component layers on two substrates. 
         FIGS. 8A-D  depict alternative configurations of force-sensitive structures. 
         FIG. 9  depicts an example of a force-sensitive component including an array of pixel elements. 
         FIG. 10  depicts a example force-sensitive structure including a pressure-sensitive adhesive and a force-sensitive component. 
         FIG. 11  depicts an example of touch sensor. 
         FIG. 12  depicts an example of a sense circuit for a touch sensor. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of examples, reference is made to the accompanying drawings in which it is shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the various examples. 
     Embodiments described herein may relate to or take the form of a force sensor that is incorporated with components of an electronic device to form a touch-sensitive surface on the device. Some embodiments are directed to a force sensor that can compensate for variations in temperature and may be optically transparent for integration with a display or transparent medium of an electronic device. Certain embodiments described herein also relate to force-sensitive structures including one or more force-sensitive components for detecting a magnitude of a force applied to a device. In one example, a transparent force-sensitive component is integrated with, or adjacent to, a display element of an electronic device. The electronic device may be, for example, a mobile phone, a tablet computing device, a computer display, a notebook computing device, a desktop computing device, a computing input device (such as a touch pad, keyboard, or mouse), a wearable device, a health monitor device, a sports accessory device, and so on. 
     Generally and broadly, a user touch event may be sensed on a display, enclosure, or other surface associated with an electronic device using a force sensor adapted to determine the magnitude of force of the touch event. The determined magnitude of force may be used as an input signal, input data, or other input information to the electronic device. In one example, a high force input event may be interpreted differently from a low force input event. For example, a smart phone may unlock a display screen with a high force input event and may pause audio output for a low force input event. The device&#39;s responses or outputs may thus differ in response to the two inputs, even though they occur at the same point and may use the same input device. In further examples, a change in force may be interpreted as an additional type of input event. For example, a user may hold a wearable device force sensor proximate to an artery in order to evaluate blood pressure or heart rate. One may appreciate that a force sensor may be used for collecting a variety of user inputs. 
     In many examples, a force sensor may be incorporated into a touch-sensitive electronic device and located proximate to a display of the device, or incorporated into a display stack. Accordingly, in some embodiments, the force sensor may be constructed of optically transparent materials. For example, an optically transparent force sensor may include at least a force-receiving layer, at least one substrate each including formed form an optically transparent material. The sensor may also include one or more force-sensitive components disposed on the substrate. In some embodiments, the substrate may be disposed below the force-receiving layer such that the force-sensitive component may experience deflection, tension, compression, or another mechanical deformation upon application of force to the force-receiving layer. 
     A transparent force-sensitive component may be formed from a compliant material that exhibits at least one measurable electrical response that varies with a deformation, deflection, or shearing of the component. The transparent force-sensitive component may be formed from a piezoelectric, piezoresistive, resistive, or other strain-sensitive material that is attached to or formed on a substrate and electrically or operatively coupled to sensor circuitry for measuring a change in the electrical response of the material. Potential substrate materials include, for example, glass or transparent polymers like polyethylene terephthalate (PET) or cyclo-olefin polymer (COP). Example transparent conductive materials include polyethyleneioxythiophene (PEDOT), indium tin oxide (ITO), carbon nanotubes, graphene, piezoresistive semiconductor materials, piezoresistive metal materials, silver nanowire, other metallic nanowires, and the like. Transparent materials may be used in sensors that are integrated or incorporated with a display or other visual element of a device. If transparency is not required, then other component materials may be used, including, for example, Constantan and Karma alloys for the conductive component and a polyimide may be used as a substrate. Nontransparent applications include force sensing on track pads or behind display elements. In general, transparent and non-transparent force-sensitive components may be referred to herein as “force-sensitive components” or simply “components.” 
     In some cases, the force-sensitive component may be placed under tension in response to a downward deflection because the component is positioned below the neutral axis of the bend of the substrate. Once under tension, the transparent force-sensitive component may exhibit a change in at least one electrical property, for example, resistance. In one example, the resistance of the transparent force-sensitive component may increase linearly with an increase in tension experienced by the component. In another example, the resistance of the transparent force-sensitive component may decrease linearly with an increase in tension experienced by the component. One may appreciate that different transparent materials may experience different changes to different electrical properties, and as such, the effects of tension may vary from embodiment to embodiment. 
     In some embodiments, the force-sensitive component is patterned into an array of lines, pixels, or other geometric elements herein referred to as “components.” The regions of the force-sensitive component or the components may also be connected to sense circuitry using electrically conductive traces or electrodes. In some cases, the conductive traces or electrodes are also formed from transparent conductive materials. In some embodiments, sense circuitry may be in electrical communication with the one or more components via the electrically conductive traces and/or the electrodes. As previously mentioned, the sense circuitry may be adapted to detect and measure the change in the electrical property or response (e.g., resistance) of the component due to the force applied. 
     In some cases, the force-sensitive components may be patterned into pixel elements, each pixel element including an array of traces generally oriented along one direction. This configuration may be referred to as a piezoresistive or resistive strain gauge configuration. In general, in this configuration the force-sensitive-component may be composed of a material whose resistance changes in a known fashion in response to strain. For example, some materials may exhibit a change in resistance linearly in response to strain. Some materials may exhibit a change in resistance logarithmically or exponentially in response to strain. Some materials may exhibit a change in resistance in a different manner. For example, the change in resistance may be due to a change in the geometry resulting from the applied strain such as an increase in length combined with decrease in cross-sectional area may occur in accordance with Poisson&#39;s effect. The change in resistance may also be due to a change in the inherent resistivity of the material due to the applied strain. 
     In some embodiments, the force-sensitive component may be formed from a solid sheet of material and is in electrical communication with a pattern of electrodes disposed on one or more surfaces of the force-sensitive component. The electrodes may be used, for example, to electrically connect a region of the solid sheet of material to sense circuitry. This configuration may be referred to as a piezoelectric-strain configuration. In this configuration, the force-sensitive component may generate a charge when strained. The force-sensitive component may also generate different amounts of charge depending on the degree of the strain. In some cases, the overall total charge is a superposition of the charge generated due to strain along various axes. 
     One or more force-sensitive components may be integrated or incorporated with a display element of a device, which may include other types of sensors. In one typical embodiment, the display element also includes a touch sensor configured to detect the location of one or more touches. A sample touch sensor is described in more detail below with respect to  FIG. 11 . Using a touch sensor and the transparent force-sensitive component in accordance with some embodiments described herein, the location and magnitude of a touch on a display element of a device can be estimated. 
     Some embodiments discussed herein are directed to various aspects of detecting force using a force-sensitive component. In one embodiment, two types of force-sensitive materials can be used to reduce or eliminate the effects due to changes in environmental conditions, such as temperature. In another embodiment, one or more reference pixels can be used to improve the measurement sensitivity of device having an array of force-sensitive components. Certain embodiments may include a pressure sensitive adhesive layer affixing various layers of the embodiment together, or affixing the embodiment to another portion of an electronic device, such as to or within a display or display stack. In such embodiments, the pressure-sensitive adhesive layer may be selected at least partially for mechanical properties that provide a desirable deflection in a force-sensitive layer. As an example, the force-sensitive component may be positioned beneath a cover glass but above an LCD layer. Since the force-sensitive component, including the individual force-sensing elements, are generally transparent, a user may see the display through the force-sensitive component even though the component is interposed between the user and display. In another example, the force-sensitive component may be positioned between a light controlling layer of an LCD and a backlight source, and therefore should be substantially transparent to allow light produced by the backlight to reach the light controlling layer. Each of these embodiments can be implemented separately or used in combination with each other, as required or desirable in a particular force-sensing configuration. 
     In general, the amount of force produced by a touch can be estimated using a force-sensitive component integrated into a device. In one typical embodiment, the force-sensitive component is integrated with, or placed adjacent to, portions of a display element of a device, herein referred to as a “display stack” or simply a “stack.” A force-sensitive component may be integrated with a display stack, by, for example, being attached to a substrate or sheet that is attached to the display stack. Alternatively, the force-sensitive component may be placed within the display stack in certain embodiments. Examples of a force-sensitive component that is integrated with a display stack are provided below, with respect to  FIGS. 2-8 . Although the following examples are provided with respect to force-sensitive component integrated with a display stack, in other embodiments, the force-sensitive component may be integrated in a portion of the device other than the display stack. 
     As mentioned previously, the force-sensitive component is configured to produce a measureable change in one or more electrical properties in response to a deflection of the force-sensitive component. For example, the force-sensitive component may produce a change in impedance or resistance in response to a deflection. This type of implementation may also be referred to as a strain-gauge component. The force-sensitive component may also or alternatively produce a current or charge in response to a deflection. This type of implementation may also be referred to as a strain-sensitive thin component transistor or piezoelectric component. 
     One challenge with using a force-sensitive component is that a given electrical property may change in response to variable environmental conditions, such as a change in temperature. In one example, an increase in temperature results in a thermal expansion of the force-sensitive component, which changes the electrical property that may be measured by the sense circuitry. An increase in temperature may occur, for example, as a result of heat produced by the display element or other electronic elements of the device. Similarly, the temperature of the force-sensitive component may decrease due to a decrease in the temperature of the ambient conditions resulting in thermal contraction of the force-sensitive component. Heating or cooling may also occur as a result of a touch if, for example, the touch is performed with a finger that is a different temperature than the force-sensitive component. The force-sensitive component may also expand and contract in response to changes in other environmental conditions, such as changes in humidity. In the following examples, the electrical property is a resistance and the variable environmental condition is temperature. However, the techniques can also be applied to different electrical properties, such as capacitance, that may be affected by changes in other environmental conditions. 
     Based on a measurement of the electrical property alone, it may be difficult to distinguish a change in an electrical property due to changing environmental condition and a change due to a deflection produced by the force of a touch. For example, a deflection may produce a reduction or increase in the resistance or impedance of the force-sensitive component depending on the type of material that is used and the mechanical nature of the deflection (e.g., compression or expansion). A thermal gradient may also produce a reduction or increase in the resistance or impedance of the force-sensitive component depending on whether the gradient is positive or negative. As a result, the two effects may cancel each other out or amplify each other resulting in an insensitive or hypersensitive force sensor. A similar reduction or increase in the resistance or impedance of the force-sensitive component could also be produced by, for example, an increase in temperature of the force-sensitive component due to heat produced by other elements of the device. Generally, compression or tension of the force-sensing elements defined on the substrate of the force-sensing component creates strain on the force-sensing elements. This strain may cause a change in resistance, impedance, current or voltage that may be measured by associated sense circuitry; the change may be correlated to an amount of force that caused the strain. Accordingly, in some embodiments the force-sensing elements on the component may be considered or otherwise operate as strain gages. 
     One solution to this problem is to use more than type of force-sensitive component, each type of force-sensitive component having a different sensitivity to a specific environmental condition. In one example, two different force-sensitive components are used, each component having a different sensitivity to temperature changes but similar sensitivity to strain. Knowing or characterizing each of the component&#39;s tendency to change a given material property (e.g., resistance), a comparative measurement can be computed that reduces or substantially eliminates the effect of temperature on the force estimation. 
     In some cases, it may be advantageous that the two different force-sensitive components are integrated in to the device in proximate location such that a deflection produced by a touch force results in approximately the same deflection in each of the force-sensitive components. 
     In one sample embodiment, two different force-sensitive components are in electrical communication with sense circuitry that is configured to measure an electrical property (e.g., resistance) of each of the force-sensitive components. In this example, the two different force-sensitive components have different temperature and/or strain sensitivities. If the both the temperature and strain characteristics of the force-sensitive components are known, the sense circuitry may be configured to compensate a measurement of the electrical property for changes in temperature. 
     In one example, the temperature characteristics (e.g., the temperature-dependent response) of the force-sensitive components can be approximated as a linear relationship. Similarly, the strain characteristics (e.g., the strain-dependent response) of the force-sensitive components can also be approximated as a linear relationship. For example, the resistance R 1  of a first force-sensitive component can be expressed as:
 
 R   1   =A   1   *ε+B   1   *T,   Equation 1
 
     where A 1  is a constant representing the change in resistivity with strain ε, and B 1  is a constant representing the change in resistivity with temperature T for the first force-sensitive component. Similarly, the resistance R 2  of a second force-sensitive component can be expressed as:
 
 R   2   =A   2   *ε+B   2   *T,   Equation 2
 
where A 2  is a constant representing the change in resistivity with strain ε, and B 2  is a constant representing the change in resistivity with temperature T for the second force-sensitive component. If the ratio of A 1  to A 2  is not equal to the ratio of B 1  to B 2 , then equations 1 and 2 are not degenerate and a unique solution for the strain E can be determined. In this case, a compensation due to temperature can be computed or determined based on the known temperature and strain characteristics of both force-sensitive components (using, for example, sense circuitry).
 
     In some cases, a component&#39;s strain characteristic (or strain-based response) and a component&#39;s temperature characteristic (or temperature-based response) can be determined by a calibration of the component under certain operating conditions. For example, the calibration may characterize the component measuring different forces produced by multiple touches, while the device is maintained at a near-constant temperature. A different calibration may also characterize the component by measuring the same force (or strain) on the device produced by multiple touches but at different component temperatures. Using the known temperature and strain characteristics of the component and the superposition principle, a component&#39;s response to strain and temperature can be differentiated. 
       FIG. 1  depicts an example electronic device  100  that may include a force sensor in accordance with some embodiments. The electronic device  100  may include a display  104  disposed or positioned within an enclosure  102 . The display  104  may include a stack of multiple elements including, for example, a display element, a touch sensor layer, a force sensor layer, and other elements. The display  104  may include a liquid-crystal display (LCD) element, organic light emitting diode (OLED) element, electroluminescent display (ELD), and the like. The display  104  may also include other layers for improving the structural or optical performance of the display, including, for example, glass sheets, polymer sheets, polarizer sheets, color masks, and the like. The display  104  may also be integrated or incorporated with a cover  106 , which forms part of the exterior surface of the device  100 . Example display stacks depicting some example layer elements are described in more detail below with respect to  FIGS. 2-8 . 
     In some embodiments, a touch sensor and or a force sensor are integrated or incorporated with the display  104 . In some embodiments, the touch and/or force sensor enable a touch-sensitive surface on the device  100 . In the present example, a touch and/or force sensor are used to form a touch-sensitive surface over at least a portion of the exterior surface of the cover  106 . The touch sensor may include, for example, a capacitive touch sensor, a resistive touch sensor, or other device that is configured to detect the occurrence and/or location of a touch on the cover  106 . The force sensor may include a strain-based force sensor similar to the force sensors described herein. 
     In some embodiments, each of the layers of the display  104  may be adhered together with an optically transparent adhesive. In other embodiments, each of the layers of the display  104  may be attached or deposited onto separate substrates that may be laminated or bonded to each other. The display  104  may also include other layers for improving the structural or optical performance of the display, including, for example, glass sheets, polarizer sheets, color masks, and the like 
       FIGS. 2A-B  depict example configurations using two force-sensitive components.  FIG. 2  depicts a stackup  200  including two force-sensitive component layers  210 ,  220  disposed on either side of a substrate  202 . In this example, the substrate  202  is an optically translucent material, such as polyethylene terephthalate (PET). As shown in  FIG. 2A , a first force-sensitive component layer  210  is formed from an array of components  212  and disposed on a first side of the substrate  202 . Similarly, a second force-sensitive layer  220  is also formed from an array of components  222  and disposed on a second side of the substrate  202  that is opposite to the first side. In this example, the force-sensing components  212 ,  222  are formed as an array of rectilinear pixel elements, although other shapes and array patterns could also be used. 
     The components  212 ,  222  are typically connected to sense circuitry  205  that is configured to detect changes in an electrical property of each of the components  212 ,  222 . In this example, the sense circuitry  205  is configured to detect changes in the resistance of the component  212 ,  222 , which can be used to estimate a force that is applied to the device (e.g., to an element above the stack depicted in  FIG. 2B ). In some cases, the sense circuitry  205  may also be configured to provide information about the location of the touch based on the relative difference in the change of resistance of the components  212 ,  222 . 
     In some embodiments, the sense circuitry  205  includes a computer processing unit and computer memory that are configured to execute computer-readable instructions. For example, the sense circuitry  205  may include circuitry that is configured to measure an electrical response of the components  212 ,  222  (e.g., a resistance, charge, voltage, current) due to the force of a touch on the sensor and/or a change in temperature. As discussed previously, the components  212 ,  222  may have different temperature and/or strain sensitivities. If the both the temperature and strain characteristics of the force-sensitive components are known and are unique, the sense circuitry  205  may be configured to compensate a measurement of the electrical property for changes in temperature. In some instances, the sense circuitry  205  may be configured to compensate for variations in temperature using, for example, the relationships described above with respect to Equations 1 and 2. The temperature-compensation may be performed using, for example, a combination of electrical (measurement) circuitry and computer-executable code stored in the computer-readable memory and executed by the computer processor. 
     The first and second force-sensitive components  210 ,  220  may be disposed on the substrate  202  using a pressure-sensitive adhesive (PSA) layer. The component may also be directly deposited on the substrate  202 . In some embodiments, each of the components are attached or deposited onto separate substrates that may be laminated or bonded to each other. In some embodiments, the first and second force-sensitive layers  210 ,  220  and the substrate  202  are attached or integrated with other elements of a display stack  230 . The display stack  230  may include a liquid crystal display (LCD), light-emitting diode (LED), organic light emitting diode (OLED) display, or other display element. The display stack  230  may also include other layers for improving the structural or optical performance of the display, including, for example, a cover glass sheet, polarizer sheets, color masks, and the like. Additionally, the display stack  230  may include a touch sensor for determining the location of one or more touches on the display. A sample touch sensor is described below with respect to  FIG. 11 . 
     As described previously, it may be advantageous to form the first force-sensitive layer  210  and the second force-sensitive layer  220  from two different materials, each having a different sensitivity to an environmental condition. For example, the first force-sensitive layer  210  may be formed from a PEDOT material having a first temperature sensitivity (e.g., temperature-based response) and the second force-sensitive layer  220  may be formed from an ITO material having a second temperature sensitivity (e.g., temperature-based response). In some cases, the first and second force-sensitive layers  210 ,  220  are both formed from a PEDOT material, but have different temperature sensitivities. For example, the first and second force-sensitive layers  210 ,  220  may be formed from two different materials having different PEDOT:SS ratios. In other cases, the first and second force-sensitive layers  210 ,  220  may be the same material, but treated chemically to change the strain or temperature-based response. For example, a PEDOT material may be chemically treated with dimethyl sulfoxide (DMSO). 
     In general, it may be desirable to obtain or calculate the relationship between the temperature of the component and the electrical property (e.g., resistance) and for both of the force-sensitive components. It may also be desirable to obtain or calculate the relationship between strain and the electrical property (e.g., resistance) and for both of the force-sensitive components. In accordance with the technique described above, knowing both the temperature characteristics (e.g., temperature response) and strain characteristics (e.g., strain response) for both of the force-sensitive components, the effects of temperature changes on the force sensor can be reduced or eliminated. In some cases, the known temperature characteristics and strain characteristics can be used to calculate or determine a compensation for the effects of temperature on force measurements performed using the stack  200 . 
       FIGS. 3A-B  depicts another example configuration using two types of force-sensitive components. In particular,  FIGS. 3A-B  depict a stackup  300  including two force-sensitive component layers  310 ,  320  of a first type disposed on either side of a first substrate  302  and two force-sensitive component layers  330 ,  340  of a second type disposed on either side of a second substrate  304 . In this example, the force-sensitive components are formed from an array of rectilinear strip elements  312 ,  322 ,  332 ,  342  that are located along either the X or Y directions with respect to a plane. The strip elements are arranged transverse (e.g., perpendicular to) each other to form a sensor grid. Similar to the example described above with respect to  FIGS. 2A-B , each of the strip elements  312 ,  322 ,  332 ,  342  are electrically connected to sense circuitry  305  that is configured to detect and measure changes to an electrical property that can be used to estimate a force that is applied to the device. In some cases, the sense circuitry  305  can also be used to provide an estimate of the location of the touch on the device. Similar to the example in  FIGS. 2A-B , the stackup  300  may be integrated with a display stack  330  having a display element. 
     In this example, the two force-sensitive layers  310 ,  320  are formed from a first material having first sensitivity to an environmental condition. For example, the two force-sensitive layers  310 ,  320  may be formed from a PEDOT material having a first sensitivity to temperature. Similarly, the other two force-sensitive layers  330 ,  340  may be formed from a second material having a second sensitivity to an environmental condition. For example, the two force-sensitive layers  330 ,  340  may be formed from another type of PEDOT material or an ITO having a second sensitivity to temperature. Similar to as described above with respect to  FIGS. 2A-B , if temperature and strain characteristics are known for both of the force-sensitive component layers  330 ,  340 , the effects of temperature changes on the force sensor can be reduced or eliminated. In some cases, the known temperature characteristics and strain characteristics can be used to calculate or determine a compensation for the effects of temperature on force measurements performed using the stack  300 . 
       FIGS. 4 and 5  depict two additional example structures for a stackup having a force-sensitive component.  FIG. 4  depicts a single force-sensitive component that is integrated in a device. More specifically,  FIG. 4  depicts a cross-sectional view of a piezoelectric component integrated into a sample stackup  400  including a display element of a device. Stackup  400  can include a display  402 , such as an LCD display, LED display, OLED display, or the like, for generating images to be displayed by the device. Stackup  400  can further include a piezoelectric component  408  coupled to display  402  by optically clear adhesive  404 . Piezoelectric component  408  is typically characterized as having an electrical property that changes as the piezoelectric component  408  is deflected or deformed. As depicted in  FIG. 4 , the piezoelectric component  408  can further include a first set of electrodes  406  and a second set of electrodes  410  formed on opposite surfaces of the component. The electrodes are used to connect portions of the piezoelectric component  408  with other elements of the system, including the sense circuitry  420 . A set of electrodes can include a single electrode or multiple electrodes. The electrodes can be formed from a transparent conductive material, such as ITO, PEDOT, or silver nanowire. Top views  416  and  418  show the shapes of any single one of electrodes  406  and  410 , respectively, as viewed from above stackup  400 . In the illustrated example, electrodes  406  and  410  can both have a shape that substantially matches that of piezoelectric component  408  and display  402  and can extend along the surfaces of piezoelectric component  408 . 
     In some examples, electrode  406  can be coupled to ground and electrode  410  can be coupled to sense circuitry  420  capable of detecting an amount of electric charge generated by piezoelectric component  408 . Sense circuitry  420  can include an amplifier  422  and capacitor  424 , as shown in  FIG. 4 , or it can include other circuit elements and components that are configured to detect the electrical response of the piezoelectric component  408  when it experiences strain. In other cases, the piezoelectric component  408  may be electrically connected to the sense circuitry  420  without the use of electrodes. For example, the piezoelectric component  408  may be connected to the sense circuitry  420  using conductive traces that are formed as part of the piezoelectric component layer. 
     With reference to  FIG. 4 , the stackup  400  can further include cover material  414  (e.g., glass, plastic, or other rigid and transparent material) coupled to piezoelectric component  408  by optically clear adhesive  412 . Since the materials above display  402  can be formed from transparent materials, images generated by display  402  can be viewed through the various layers of stackup  400 . 
     During operation, as a user applies a downward force on cover material  414 , cover material  414  can deform by an amount corresponding to an amount of the applied force. The deformation of cover material  414  can cause a corresponding deformation in optically clear adhesive  412  and piezoelectric component  408 . The piezoelectric component  408  may then exhibit a change in an electrical property and/or produce an electrical response due to the deformation. 
     For example, the piezoelectric component  408  may generate an electric charge based on the amount of deformation of the component. In this case, the generated electric charge may be received by sense circuitry  420  via electrode  410 . Since the amount of electric charge generated by piezoelectric component  408  may correspond to the amount of deformation of the component and because the amount of deformation of the component may correspond to the force applied to cover material  414 , the amount of electric charge detected by sense circuitry  420  may be indicative of the magnitude of the force applied to cover material  414 . 
     In some embodiments, the component  408  may be formed from a material that changes resistance due to deformation caused by the downward force on the cover material  414 . Since change in resistance of the component  408  may correspond to the amount of deformation of the component and because the amount of deformation of the component  408  may correspond to the force applied to cover material  414 , a change in impedance detected by sense circuitry  420  may be indicative of the magnitude of the force applied to cover material  414 . Thus, the sense circuitry  420  may be configured to detect a change in charge and/or resistance, which may be used to compute or estimate an amount of force applied to cover material  414 . 
       FIG. 5  depicts more than one force-sensitive component integrated into a device. In particular,  FIG. 5  depicts a cross-sectional view of a sample stackup  500  having two strain-sensitive components integrated into a device. The stackup  500  may be used, for example, to compensate for changes in the temperature of the piezoelectric components. Stackup  500  can also include a display  502 , such as an LCD, LED display, OLED display, or the like, for generating images to be displayed by the device. Stackup  500  can further include a first strain-sensitive component  508  coupled to display  502  by optically clear adhesive  504 . Stackup  500  can further include a second strain-sensitive component  512  coupled to first strain-sensitive component  508 . As discussed above, it may be advantageous that the first strain-sensitive component  508  and the second strain-sensitive component  512  have different sensitivities to an environmental condition, such as temperature. 
     In one example, the first and second strain-sensitive components  508  and  512  can both include a transparent component capable of generating a change in resistance or charge in response to a deformation of the component. In another example, the first and second strain-sensitive components  508  and  512  may change impedance due to deformation caused by the downward force. Similar to as described above with respect to  FIG. 4 , the sense circuitry  520  can be used to detect and measure changes in an electrical property of the components. The changes in the electrical property can be used to calculate and estimate of the force that is applied to the device. 
     Similar to as described above, if temperature and strain characteristics are known for both of the force-sensitive component layers  508 ,  512 , the effects of temperature changes on the force sensor can be reduced or eliminated. In some cases, the known temperature characteristics and strain characteristics can be used to calculate or determine a compensation for the effects of temperature on force measurements performed using the stack  500 . 
       FIGS. 6 and 7  depict two other examples of stacks  600  and  700 , respectively. Similar to the examples provided above, the stacks may incorporate at least two force-sensitive components that are formed from a different type of force-sensitive material having different sensitivities to a specific environmental condition and/or strain characteristics. Accordingly, the stacks  600  and  700  can also be used to produce a force sensor that is configured to compensate for changes in environmental conditions. 
     As shown in  FIG. 6 , the stack  600  includes a first layer  606 , which is formed from an array of force-sensitive structures (traces) formed on a surface of a substrate  608 . In some embodiments, the first layer  606  may be formed from a PEDOT material and the substrate  608  may be formed from a PET material. On the opposite side of the substrate  608  the layer the stack includes another array of force-sensitive structures (traces) formed on a respective surface of the substrate  608 . In some embodiments, the force-sensitive structures of each array are configured to detect strain along different directions. For example, the first layer  606  may be formed from structures that are configured to sense strain along a first direction and the second layer  601  may be formed from structures that are configured to sense strain along a second direction that is perpendicular to the first direction. 
     As shown in  FIG. 6 , stack  600  also includes a third layer  614  disposed on a surface of a second substrate  616 . The third layer  614  may include an array of force-sensitive structures (traces) formed form a material that is different than the first layer  606  and the second layer  610 . In some embodiments, the third layer  614  is formed from an ITO material and the second substrate  616  may be formed from a PET material. As discussed previously, if two or more layers are formed from materials having different temperature and/or strain characteristics, the electrical response or property of the two structures may be used to compute a force estimate that compensates for variations in temperature. 
     The stack  600  depicted in  FIG. 6  may be formed by the following example process. The stack  600  may be formed by, for example, patterning the third layer  614  (e.g., an ITO layer) on one side of the second substrate  616 . The two other layers  606  and  610  (e.g., PEDOT layers) may be patterned on opposite sides of the first substrate  608 . A first adhesive layer  612  may be used to bond the first substrate  608  to the second substrate  616 . Similarly, a second adhesive layer  604  may be used to bond the sensor stack to other components of the display. In the present example, the second adhesive layer  604  bonds the first substrate  608  to a surface of the rear polarizer  602  of a display. The adhesive layers may be formed form an optically clear adhesive, pressure sensitive adhesive, or other suitable bonding material. 
     As shown in  FIG. 6 , an electrical connector  620  may be formed on a portion of the substrates. In particular, a first metal conductive layer  622  may be formed on a surface of the first substrate  624  and electrically coupled to structures of the first layer  606 . A second metal conductive layer  624  may be formed on an opposite surface of the first substrate  624  and electrically coupled to the structures of the second layer  610 . A third metal conductive layer  626  may be formed on a surface of the second substrate  616  and electrically coupled to the structures of the third layer  614 . The metal conductive layers  622 ,  624 , and  626  may be formed from a silver or other electrically conductive material. 
       FIG. 7  depicts another embodiment of a stack  700  having force-sensing structures formed from different materials. Stack  700  is similar to stack  600  except that the electrical connector  720  is formed by attaching electrical conduits  722 ,  724 ,  726  are electrically connected to the sensor layers by electrical connections  732 ,  734 ,  730 , respectively. The electrical conduits  722 ,  724 ,  726  may be formed as a flexible circuit or other type of flexible conduit. Similar to the previous example, a first layer  706  and second layer  710  are formed on opposite sides of a first substrate  708 . The first layer  706  and second layer  710  may be formed from a PEDOT material. A third layer  714  may be attached to a second substrate  716 . The third layer  714  may be formed form a different material, such as ITO. The first substrate  708  may be bonded to the second substrate  716  by a first adhesive layer  712 . Similarly, first substrate  708  may be bonded to another layer of the display stack, such as the rear polarizer  702  by a second adhesive layer  704 . 
       FIGS. 8A-D  depict alternative configurations of a force sensor having multiple types of force-sensitive component layers.  FIG. 8A  depicts an example stack  810  formed from two PEDOT layers  820 ,  824  disposed on either side of a single substrate  822 . Each PEDOT layer  820 ,  824  may be formed from one or more sets of force-sensitive components (e.g., traces) generally oriented along a direction in the plane of the layer. In some cases, the traces of the two PEDOT layers  820 ,  824  are substantially perpendicular to each other, but otherwise may be oriented along any direction in the plane of the layer. The PEDOT layers are then passivizated forming passivation layers  818  and  826  and two ITO layers  816 ,  828  are formed on top of the respective passivation layers  818 ,  826 . The ITO layers  816 ,  828  may also be formed from traces oriented in a direction in the plane of the layer and the traces of the two layers are typically substantially perpendicular to each other. An adhesive layer  814  (e.g., an OCA) can be used to bond the force-sensitive layers to the rear polarizer  812  of a display. 
       FIG. 8B  depicts a similar configuration  8300 , but with two ITO layers  840 ,  844  deposed on either side of the substrate  842 . In a similar fashion, the ITO layers  840 ,  844  are passivated and two PEDOT layers  836 ,  848  are formed on top of the respective passivation layers  838 ,  846 . The force sensitive-structure may be bonded to or formed on a surface of the rear polarizer  834  of a display. 
       FIG. 8C  depicts an example stack  850  having layers that are formed from two materials in the same layer. For example, a first layer  856  may be formed from ITO components  857  and PEDOT components  858  formed on the surface of a substrate  862 . Similarly, a second layer  864  may be formed from ITO components  865  and PEDOT components  866 . In the present embodiment, the ITO and PEDOT components of each layer may alternate. However, the specific arrangement of the ITO and PEDOT layers may vary depending on the application. The force-sensing structure may be attached to other layers of a display stack by an adhesive layer  854 . 
       FIG. 8D  depicts an example stack  870  having layers that are formed from materials having different properties in the same layer. For example, a first layer  876  may be formed from two types of PEDOT material. In some embodiments, the first layer  876  may be formed from alternating PEDOT components  887  and PEDOT components  888  having different PEDOT:SS ratios formed on a surface of the substrate  882 . In some embodiments, the alternating PEDOT components  887  and PEDOT components  888  may be chemically treated to produce different temperature and/or strain sensitivities. Similarly, a second layer  884  may be formed from alternating PEDOT components  885  and PEDOT components  886  having different temperature and/or strain sensitivities. The force-sensing structure may be attached to other layers of a display stack by an adhesive layer  874 . 
     As described with respect to the embodiments above, the amount of deflection can be estimated based on a measured electrical property of the force-sensitive component. In some cases, the changes in the electrical property to be measured are relatively small over normal operating conditions. For example, the change in resistance due to a deflection caused by an applied force may be very small and may vary less than changes in resistance due to normal temperature variations. In this case, a direct resistance measurement may be difficult to measure and may be susceptible to noise or other measurement artifacts. 
     Thus, in some cases, it may be advantageous to measure an electrical property of one component region using one or more reference component regions in order to increase the sensitivity of the measurement and reduce the effects of noise or other measurement artifacts. Using a reference component region may also help to reduce the effects of normal temperature variations on a resistance measurement. 
     In one example, multiple pixel regions are formed the force-sensitive component. The pixel regions may form a grid of sensor pixels that can be used to detect deflection over a respective pixel area. In this case, one or more pixels may be used as a reference pixel to increase the sensitivity of the measurement of the electrical property of a primary pixel that is being measured. The following example is provided with respect to a measurement of the resistance of a pixel. However, in other embodiments, another type of electrical property may be measured using a similar technique. 
       FIG. 9  depicts an example of a force-sensitive component layer that may be integrated into a device (e.g., device  100  of  FIG. 1 ). As shown in  FIG. 9 , the force-sensitive component layer  900  may be formed from an array of pixel elements  901  arranged along the X and Y axes on a substrate  905 . Each of the pixels may be formed from a piezo-resistive or resistive material that changes in resistance due to a deflection in the component. Other types of force-sensitive component materials having other force-sensitive properties could also be used. 
     In this example, every other pixel element  901  in the array is configured to serve as a reference pixel for each other. That is, for any pixel measurement in the array, there is a primary pixel and a reference pixel, where the primary pixel and the reference pixel are separated by at least one other pixel. A measurement of each of the pixels  901  may be performed with each of the pixels being sequentially designated as the primary pixel. 
     With regard to  FIG. 9 , an example primary pixel is designated as  910 . When measuring the electrical property (e.g., resistance) of the primary pixel, one or more reference pixels (e.g.,  911 ,  912 ) can be used to improve the sensitivity of the measurement. In this example, resistance measurement is taken both at the primary pixel  910  and one or more of the reference pixels ( 911 ,  912 ). The resistance measurement may be taken using the example conductive traces depicted in  FIG. 9  ( 921 ,  922 ,  923 ,  924 ). Conductive traces to the other pixels  901  in the array are not shown for clarity. Also, other conductive trace configurations may be used to measure the resistances of the pixels in the array. 
     In this example, sense circuitry  903  is configured to bridge the resistances of the primary and one or more of the reference pixels using, for example, a Wheatstone Bridge circuit configuration. For example, the sense circuitry  903  may monitor or measure the voltage of a half bridge formed between one reference pixel  911  and the primary pixel  912 . Another half bridge could be formed between the other reference pixel  912  and the primary pixel  912 . In this example, the sense circuitry  903  is configured to balance the changes in the resistance between the two half bridges to increase the sensitivity of measurement. That is, small changes in resistivity of the primary pixel can be more easily detected using the balanced bridge configuration. Furthermore, the bridge configuration may also compensate for variations in resistance due to normal variations in temperature because the primary and reference pixels may all be subjected to approximately the same temperature conditions. 
     As mentioned above, it may be advantageous to select a reference pixel that is separated from the primary pixel by at least one other pixel. Using this technique may result in a reference measurement that is more likely to be taken at the same environmental conditions but less likely to be subjected to the same deflection condition. For example, a reference pixel that is separated from the primary pixel by at least one other pixel is close enough that it will likely be at a temperature that is very similar to the primary pixel. Additionally, because the reference pixel is at least one pixel away from the primary pixel, it is likely that it will not be subjected to the same deflection conditions. 
     In some embodiments, a second array of pixel elements is disposed on the opposite side of the substrate  905 . The second array of pixel elements may also be arranged along the X and Y axes similar to the first array of pixel elements  901  depicted in  FIG. 9 . In one example, the first array of pixel elements are formed from an array of traces generally oriented along an X-axis forming an X-strain gauge element. The second array of pixels may be formed from an array of traces generally oriented along a Y-axis forming a Y-strain gauge element. In some embodiments, the second array of pixel elements may be staggered or offset with respect to the first array of pixel elements. 
     In this configuration, both first and second pixel arrays may be used to perform force detection in situations where multiple touches are employed. For example, if a first touch is generally located at the primary pixel  910  and a second touch is located at one of the reference pixels ( 911 ,  912 ) a half bridge sensing configuration may not detect either touch because the resistance of both the primary and reference pixel will change due to the two touches canceling out the measured change in resistance. In this case, because the pixel elements of the second array are offset of staggered with respect to the first array of pixel elements, a bridge formed between the primary and reference pixel of the second array will likely not be canceled and may be used to detect the two touches. 
     In one exemplary embodiment, a group of four alternating pixel elements may be connected to form a full bridge. In this case, it may be advantageous that every other pixel element be formed from an array of traces generally oriented along either the X or Y axis. That is, the pixel elements in the array alternate between being oriented along the X and Y axes. In the case of two touches, each touch on a primary and reference pixel element of a bridge, differences in the X-direction strain and the Y-direction strain may be used to detect the two touches. This effect may be more pronounced if the pixel elements are rectangular in shape and alternating in orientation by 90 degrees along a row or column of the array. This configuration may provide improved noise rejection and better temperature compensation as compared to a two-layer configuration. 
     The examples provided above are directed to pixel elements formed from an array of conductive traces generally oriented along either the X-axis or Y-axis to detect either an X strain or a Y strain respectively. However, in some embodiments, the conductive traces may be generally oriented at an angle with respect the X or Y axis to detect strain along a variety of different directions. Depending on the structural and mechanical characteristics of the sensor, the direction of the maximum strain may be locate at an angle with respect to the X or Y axes. In this case, it may be advantageous to orient the traces of the pixel elements to measure strain along the direction of maximum strain. 
     Although the examples described above are drawn to a rectilinear array of pixel elements, other pixel configurations could also be used. For example, a polar array or other geometric arrangement of pixel elements may be formed on one or both sides of a substrate. Also, more than one pixel element may separate the primary pixel from the reference pixel, depending on the geometry of the pixel array. Also, one, two, or more than two reference pixel elements may be used to measure the electrical property of the primary pixel element. 
     Because the magnitude of the measurements are relatively small, in some embodiments, the sense circuitry  903  includes a low noise amplifier, such as a programmable gain amplifier (PGA) to help detect and measure changes in resistance of the pixel elements  901 . The PGA may also be connected to a delta-sigma analog-to-digital converter (ADC) and a digital filter to improve the performance of the sense circuitry  903 . The sense circuitry  903  may contain multiple PGA, ADC, and filter elements to detect and measure changes in resistance over the entire array of pixel elements  901 . In some cases, a multiplexer is used to combine conductive lines from multiple pixel elements  901  into a single PGA and ADC. This is one example, and other sensing configurations may also be employed for the sense circuitry  903   
     To improve the performance of the sensor, a ground shield layer having low impedance to ground may also be used. The use of a ground shield layer may reduce or eliminate noise due to low-magnitude voltage variations that may interfere with the measurement of the pixel elements. In one embodiment, a ring of silver material is formed around the perimeter of the ground shield layer to help to shield the array from noise. 
     In some embodiments, a differential measurement scheme may be used to reduce non-uniformity error in the array of pixel elements. Non-uniformity error may be caused by, for example, variations in the structural and mechanical characteristics of the sensor over the entire area. For example, a force applied at the top-center of the sensor may produce a different strain profile that than a fore applied at the absolute center of the sensor. To reduce the effect of these types of non-uniformities, a differential measurement may be taken at a location that is offset from the location of the touch and used to reduce the variation in the measurement due to the non-uniformity. 
     In addition, the strain response of a sensor may vary due to manufacturing tolerances and variability in the properties of the materials. In some cases, an array of pixel elements can be measured at the time of manufacture and a set of calibration coefficients can be calculated and stored. The set of calibration coefficients may be used to reduce the variability of a sensor response due from device to device. 
     In accordance with some embodiments described herein, a force-sensitive component or structure may be integrated with a display element or other element of a device using a bonding agent, such as a pressure sensitive adhesive (PSA) layer. Depending on the stackup configuration, the mechanical properties of the pressure sensitive component may affect the sensitivity and performance of a force-sensitive component. 
       FIG. 10  depicts a sample stackup  1000  having a force-sensitive component  1010  attached to a display stack  1001  by a PSA layer  1005 . In this example, a force that is applied to the display stack  1001  is transmitted through the PSA layer  1005  to the force-sensitive component  1010 . 
     In one example, the PSA layer  1005  may be relatively elastic and have an elasticity of, for example, approximately 0.3 MPa. In this case, the deflection in the force-sensitive component  1010  may be reduced because of the compliance in the PSA layer  1005 . This may reduce the sensitivity of the device, which may limit the lower bound of the force that can be practically and reliably measured. However, using a PSA layer  1005  that is relatively elastic may also be beneficial because it may result in a deflection of the force-sensitive component  1001  that is more desirable from another aspect. For example, the deflection may be more evenly distributed across the force-sensitive component  1001 , which may be advantageous in some circumstances. 
     In one example, the PSA layer  1005  may be relatively inelastic and have an elasticity of, for example, approximately 1000 MPa. In this case, the deflection in the force-sensitive component  1010  may be increased because of the reduced compliance in the PSA layer  1005 . This may increase the sensitivity of the device, which will improve the ability of the device to detect a relatively small applied force. However, using a PSA layer  1005  that is relatively inelastic may be less desirable in certain cases. For example, the deflection may be too great in areas resulting in clipping or plateauing of the measurement that can be taken of the force-sensitive component  1001 . 
     In some cases, the PSA layer  1005  may have a medium elasticity that is a balance of the two examples provided above. For example, the PSA layer  1005  may have an elasticity of approximately 1 MPa. In this case, the PSA layer  1005  may be elastic enough to produce a smooth predictable deflection in the force-sensing component  1001  without resulting in plateauing or clipping of the measurement. 
       FIG. 11  illustrates a sample touch sensor  1100  according to some embodiments of the disclosure. Touch sensor  1100  can include an array of touch nodes  1106  that can be formed by a two-layer electrode structure separated by a dielectric material. One layer of electrodes can comprise a plurality of drive lines  1102  positioned substantially perpendicular to another layer of electrodes which can comprise a plurality of sense lines  1104 , with each of the nodes  1106  having an associated mutual capacitance  1114  (also referred to as coupling capacitance). The drive lines  1102  and sense lines  1104  cross over each other in different planes separated from one another by a dielectric. Alternatively, in other embodiments the drive lines  1102  and sense lines  1104  can be formed by a one-layer electrode structure. 
     Drive lines  1102  (also referred to as rows, row traces, or row electrodes) can be activated by a stimulation signal provided by respective drive circuits  1108 . Each of the drive circuits  1108  can include an alternating current (AC) voltage source referred to as a stimulation signal source. To sense touch event(s) on the touch sensor  1100 , one or more of the drive lines  1102  can be stimulated by the drive circuits  1108 , and the sense circuitry  1110  can detect the resulting voltage values from the sense lines  1104 . The voltage values can be indicative of a finger or object altering charge from the mutual capacitance signal. The detected voltage values can be representative of node touch output values, with changes to those output values indicating the node locations  1106  where the touch events occurred and the amount of touch that occurred at those location(s). 
       FIG. 12  illustrates a sample sense circuit  1200 , which is an example of the sense circuit  1110  of  FIG. 11 . Drive circuit  1108  can produce drive signals (also referred to as stimulation signals V stim ), which can be transmitted on drive lines  1102  that contain a line resistance  1218  and coupled onto sense lines  1104  due to mutual capacitance  1114  (referred to as C sig ) between the drive and sense lines. The coupled signal can then be received by sense amplifier  1214 . Sense amplifier  1214  can include operational amplifier  1202 , and at least one of a feedback resistor  1206  and a feedback capacitor  1204 .  FIG. 12  is shown for the general case in which both resistive and capacitive feedback elements are utilized. The signal can be inputted into the inverting input (referred to as V in ) of the operational amplifier  1202 , and the non-inverting input can, in some embodiments, be tied to a reference voltage V ref  at  1208 . If V stim  is a sinusoidal signal (such as an AC signal), the output of the amplifier, V out , should also be a sinusoid. Moreover, V out  should be a sinusoid that possesses the same frequency as V stim  with a phase shift. For example, if
 
 V   stim   =A  sin(ω t ), then  Equation 3
 
 V   out   =B  sin(ω t +φ),  Equation 4
 
where φ is the phase shift. The value of φ can be influenced by many factors, including any parasitic capacitance  1216  (C par ) encountered by the sense circuit  1200 . Parasitic capacitance  1216  can be characterized as any capacitance other than the mutual capacitance  1114  between the drive lines  1102  and sense lines  1104  which is the capacitance of interest. The parasitic capacitance may be connected in series with G sig  as shown at  1216   c  and  1216   d  or may alternatively be connected in parallel as shown at  1216   a  or  1216   b . The number  1216  is used to represent any one or more of the parasitic capacitances  1216   a - 1216   d . There can be multiple factors that contribute to the value of parasitic capacitance  1216  including coupling with metallic elements within the display and variations in the air gap or other resilient members of the stack up. As shown in  FIG. 12 , V out  can then be heterodyned by being fed into a multiplier  1210 , and multiplied with a local oscillator  1212  to produce V detect    1222 . The direct current (DC) portion of V detect    1222  can be used to detect if a touch or proximity event has occurred.
 
     While the present disclosure has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the disclosure is not limited to them. Many variations, modifications, additions, and improvements are possible. More generally, embodiments in accordance with the present disclosure have been described in the context of particular embodiments. Functionality may be separated or combined in procedures differently in various embodiments of the disclosure or described with different terminology. These and other variations, modifications, additions, and improvements may fall within the scope of the disclosure as defined in the claims that follow.

Metadata:
Filing Date: 20170526
Publication Date: 20190924
Grant Date: 20190924
Priority Date: 20140113
Inventors: FILIZ, SINAN
PEDDER, JAMES E.
OGATA, CHARLEY T.
SMITH, JOHN STEPHEN
PATEL, DHAVAL CHANDRAKANT
CHOI, SHIN JOHN
HUPPI, BRIAN Q.
BUTLER, CHRISTOPHER J.
GRUNTHANER, MARTIN P.
Assignee: APPLE INC
CPC Classifications: [{"code": "G06F3/045", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L1/205", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04111", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L1/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04103", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L1/16", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/205", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0412", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01L1/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/045", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0412", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/16", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/205", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0416", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04111", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04103", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L1/18", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/04144", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/18", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/045", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/04144", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/18", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/044", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 52434986