PATENT DOCUMENT

Publication Number: US-9791958-B2
Application Number: US-201715403034-A
Country: US
Kind Code: B2

Title: Electronic device with noise-cancelling force sensor

Abstract:
An electronic device may have a housing in which components such as a display are mounted. A strain gauge may be mounted on a layer of the display such as a cover layer or may be mounted on a portion of the housing or other support structure. The layer of material on which the strain gauge is mounted may be configured to flex in response to pressure applied by a finger of a user. The strain gauge may serve as a button for the electronic device or may form part of other input circuitry. A differential amplifier and analog-to-digital converter circuit may be used to gather and process strain gauge signals. The strain gauge may be formed form variable resistor structures that make up part of a bridge circuit that is coupled to the differential amplifier. The bridge circuit may be configured to reduce the impact of capacitively coupled noise.

Claims:
What is claimed is: 
     
       1. An electronic device, comprising:
 a display; 
 a display cover layer, wherein the display cover layer comprises first and second ends; 
 a strain gauge having first and second variable resistors formed on an interior surface of the display cover layer; and 
 a differential amplifier that receives signals from the first and second variable resistors, wherein the first and second variable resistors are located at the second end of the display cover layer, and wherein the first and second variable resistors are configured to serve as an input device for the electronic device. 
 
     
     
       2. The electronic device defined in  claim 1 , wherein the input device is a pressure-actuated input device. 
     
     
       3. The electronic device defined in  claim 1 , wherein the input device is an input device selected from the group consisting of: a menu button, a volume button, a power button, a button with more than one function, a keyboard key, and a sliding input-output device. 
     
     
       4. The electronic device defined in  claim 1 , wherein the input device is an input device selected from the group consisting of: a menu button, a volume button, a power button, and a button with more than one function. 
     
     
       5. The electronic device defined in  claim 1 , further comprising a speaker port located at the first end of the display cover layer. 
     
     
       6. The electronic device defined in  claim 1 , wherein the first and second variable resistors have different shapes. 
     
     
       7. The electronic device defined in  claim 1 , wherein the first and second variable resistors have different sizes. 
     
     
       8. The electronic device defined in  claim 1 , wherein the input device is sensitive to pressure from a user&#39;s finger. 
     
     
       9. A cellular telephone, comprising:
 a housing; 
 a display in the housing that forms a front face of the cellular telephone; 
 a speaker port formed on the front face of the cellular telephone; 
 a data port formed in the housing; 
 a display cover layer; 
 a strain gauge having first and second variable resistors formed on an interior surface of the display cover layer; and 
 a differential amplifier that receives signals from the first and second variable resistors, wherein the display cover layer has first and second opposing ends, wherein the first and second variable resistors are located at the second end of the display cover layer, and wherein the first and second variable resistors are configured to serve as an input device for the cellular telephone. 
 
     
     
       10. The cellular telephone defined in  claim 9 , wherein the input device is a pressure-actuated input device. 
     
     
       11. The cellular telephone defined in  claim 9 , wherein the input device is an input device selected from the group consisting of: a menu button, a volume button, a power button, a button with more than one function, a keyboard key, and a sliding input-output device. 
     
     
       12. The cellular telephone defined in  claim 9 , wherein the input device is an input device selected from the group consisting of: a menu button, a volume button, a power button, and a button with more than one function. 
     
     
       13. The cellular telephone defined in  claim 9 , wherein the first and second variable resistors are adjacent to each other on the display cover layer.

Description:
This application is a continuation of U.S. patent application Ser. No. 15/005,732, filed Jan. 25, 2016, which is a continuation of U.S. patent application Ser. No. 13/329,133 filed Dec. 16, 2011, which are hereby incorporated by reference herein in their entireties. This application claims the benefit of and claims priority to patent application Ser. No. 15/005,732, filed Jan. 25, 2016, and patent application Ser. No. 13/329,133, filed Dec. 16, 2011. 
    
    
     BACKGROUND 
     This relates to electronic devices and, more particularly, to electronic devices with strain gauges. 
     A strain gauge can be used as part of an input device to gather user input. If care is not taken, system noise such as capacitively coupled noise from a user&#39;s body may degrade strain gauge performance. Poorly performing strain gauges may give rise to erroneous strain gauge measurements. 
     It would therefore be desirable to be able to provide improved strain gauges for electronic devices. 
     SUMMARY 
     An electronic device may have a housing in which components such as a display are mounted. The display may be covered by a display cover layer. The display cover layer may be formed from glass, plastic, or other transparent material. 
     A strain gauge may be mounted on the display cover layer, on a portion of the housing, or on other support structures within the electronic device. For example, the strain gauge may be formed from patterned lines on an interior surface of the display cover layer. The strain gauge may be used to form a button or other input device for the electronic device. 
     The layer of material on which the strain gauge is mounted may be configured to flex in response to pressure applied by a finger of a user. The strain gauge may have a differential amplifier that produces output in response to the applied pressure. 
     The strain gauge may be formed from variable resistor structures that make up part of a bridge circuit that is coupled to the differential amplifier. The bridge circuit may be configured to reduce the impact of noise. For example, structures in the bridge circuit such as the variable resistor structures may have sizes (areas), shapes, and locations that are configured to equalize how much noise appears across the differential inputs of the differential amplifier. By equalizing the amount of noise between positive and negative differential inputs to the differential amplifier, noise on the output of the differential amplifier may be minimized. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device with a strain gauge in accordance an embodiment of the present invention. 
         FIG. 2  is a cross-sectional side view of an electronic device structure such as a display cover layer or housing structure on which a strain gauge has been mounted in accordance with an embodiment of the present invention. 
         FIG. 3  is a cross-sectional side view of the strain gauge of  FIG. 2  in a deflected configuration in accordance with an embodiment of the present invention. 
         FIG. 4  is a top view of a portion of an illustrative strain gauge element such as a variable resistor in accordance with an embodiment of the present invention. 
         FIG. 5  is a diagram of an electronic device having a force sensor formed from a pair of force sensitive elements in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagram of illustrative strain gauge circuitry in accordance with an embodiment of the present invention. 
         FIG. 7  is a diagram of an illustrative strain gauge circuit having a strain gauge element formed from a variable resistor in accordance with an embodiment of the present invention. 
         FIG. 8  is a diagram of an illustrative strain gauge circuit having a pair of strain gauge elements formed from variable resistors in accordance with an embodiment of the present invention. 
         FIG. 9  is a diagram of an illustrative strain gauge circuit having four strain gauge elements in accordance with an embodiment of the present invention. 
         FIG. 10  is a top view of a portion of an electronic device having a strain gauge formed from differential strain gauge circuitry in accordance with an embodiment of the present invention. 
         FIG. 11  is a diagram showing how noise coupling on strain gauge elements for the first and second arms of a differential strain gauge may be balanced using side-by-side meandering path elements of equal area in accordance with an embodiment of the present invention. 
         FIG. 12  is a diagram showing how noise coupling on strain gauge elements for the first and second arms of a differential strain gauge may be balanced using meandering path elements that follow the same path and run parallel to each other in accordance with an embodiment of the present invention. 
         FIG. 13  is a diagram showing how noise coupling on strain gauge elements for the first and second arms of a differential strain gauge may be balanced using structures that overlap each other on a substrate in accordance with an embodiment of the present invention. 
         FIG. 14  is a diagram showing how noise coupling on strain gauge elements for the first and second arms of a differential strain gauge may be balanced using structures with different shapes but equal areas in accordance with an embodiment of the present invention. 
         FIGS. 15 and 16  are diagrams showing how noise coupling on strain gauge elements for the first and second arms of a differential strain gauge may be balanced using coupling pad structures in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as device  10  of  FIG. 1  may be provided with one or more strain gauges. A strain gauge in a device such as device  10  may be used to gather user input. For example, a strain gauge may be used as a user input device that is sensitive to pressure from a user&#39;s finger or other external object. 
     Strain gauge structures may be mounted to an exposed device surface such as a portion of a display, a housing sidewall, other housing structures such as a rear housing wall structure, or other device structure. With this type of mounting location, strain gauge structures may be used to implement input devices that lie flush with an exposed surface of device  10 . 
     Examples of input devices that may be formed using strain gauge structures include menu buttons, volume buttons, power buttons, buttons with one or more other functions, keyboard keys, sliding input-output devices (e.g., sliders for continuous volume adjustment or other control functions), and other pressure-actuated input devices. The use of a strain gauge to implement a button is sometimes described herein as an example. If desired, strain gauge structures may be used in implementing other input structures in device  10 . The use of a strain gauge to form a button is merely illustrative. 
     Device  10  of  FIG. 1  may be a portable computer, a tablet computer, a computer monitor, a handheld device, global positioning system equipment, a gaming device, a cellular telephone, a desktop computer, a computer built into a computer monitor, a television, a set-top box, or other electronic equipment. 
     Device  10  may include a housing such as housing  12 . Housing  12 , which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of these materials. 
     Housing  12  may be formed using an unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). 
     In some configurations, housing  12  may be formed using front and rear housing structures that are substantially planar. For example, the rear of device  10  may be formed from a planar housing structure such as a planar glass member, a planar plastic member, a planar metal structure, or other substantially planar structure. The edges (sidewalls) of housing  12  may be straight (vertical) or may be curved (e.g., housing  12  may be provided with sidewalls formed from rounded extensions of a rear planar housing wall). 
     As shown in  FIG. 1 , the front of device  10  may include a display such as display  14 . Display  14  may, for example, be a touch screen that incorporates capacitive touch electrodes or a touch sensor formed using other types of touch technology (e.g., resistive touch, light-based touch, acoustic touch, force-sensor-based touch, etc.). Display  14  may include pixels formed from light-emitting diodes (LEDs), organic LEDs (OLEDs), plasma cells, electronic ink elements, liquid crystal display (LCD) components, or other suitable display pixel structures. 
     Device  10  may include input-output ports, buttons, sensors, status indicator lights, speakers, microphones, and other input-output components. As shown in  FIG. 1 , for example, device  10  may include one or more openings in display  14  such as an opening to accommodate speaker port  18 . One or more openings in device  10  may also be formed in housing  12  (e.g., to accommodate input-output ports such as data port  20 ). 
     Strain gauge structures may be used in forming input-output components for device  10 . For example, one or more buttons or other input devices may be formed from strain gauges in device  10 . In the illustrative example of  FIG. 1 , stain gauge  16  has been used to form a button such as a menu on the lower portion of the front face of device  10  (e.g., in a lower end portion of display  14 ). If desired, other locations may be used for forming buttons and other input devices from strain gauge structures. The use of strain gauge  16  to form a button on display  14  of device  10  is merely illustrative. 
     Force sensors in device  10  such as strain gauge  16  may be formed from variable resistors. If desired, other types of force sensors may be formed using other types of transducers. For example, force sensors in device  10  may be formed from transducers such as variable capacitors, variable inductors, or other structures that are capable of producing an output signal that is responsive to strain or force. Arrangements in which a force sensor (i.e., strain gauge  16 ) is formed using variable resistors are sometimes described herein as an example. This is, however, merely illustrative. Force sensors in device  10  such as strain gauge  16  may be formed from any suitable transducers if desired. 
     A cross-sectional side view of an illustrative strain gauge structure is shown in  FIG. 2 . As shown in  FIG. 2 , strain gauge  16  may include strain gauge structures such as strain gauge structures  24  that are mounted to a device structure such as structure  22 . Strain gauge structures  24  may be, for example, variable resistor structures that are formed from a patterned material (e.g., a metal or other material that is deposited using physical vapor deposition, chemical vapor deposition, or other deposition techniques and that is patterned using photolithography or other patterning techniques). 
     The material from which strain gauge element  24  is formed may be patterned metal (e.g., platinum-iridium, platinum-tungsten, copper nickel alloys such as constantan, alloys of iron, nickel, and chromium, other metal alloys, or other thin-film materials such as indium tin oxide, etc.). Stain gauge  16  may be mounted on part of display  14  or part of housing  12  or may be formed from other structures in device  10 . As one example, structure  22  for strain gauge  16  may be formed from a display cover layer. The display cover layer may be a layer of glass that forms the outermost layer of display  14  or a plastic layer that covers display  14 . If desired, structures such as structure  22  may be formed from other planar sheets of material associated with display  14  (e.g., a color filter layer, polarizer layer, thin-film transistor layer, substrate layer, etc.). 
     Structure  22  may have opposing first and second surfaces such as surface  22 A and surface  22 B. Surface  22 A may be an outer surface and surface  22 B may be inner surface. For example, surface  22 A may be the outer surface of a display cover layer in display  14  and surface  22 B may be the inner surface of the display cover layer. Strain gauge element  24  may be located on surface  22 B (e.g., an inner display surface or housing surface) in the example of  FIG. 2 , but may be located on surface  22 A, if desired. 
     As shown in  FIG. 3 , strain gauge  16  may be placed in a strained condition when a user&#39;s finger or other external object such as object  26  of  FIG. 3  is pressed downwards (inwards) on surface  22 A of structure  22 . By exerting pressure on strain gauge  16  (i.e., by pressing on outer surface  22 A of strain gauge support structure  22 ), structure  22  may be flexed inwardly as shown in  FIG. 3 . In this flexed configuration, strain gauge structures  24  may be bent. 
     The process of bending strain gauge structures  24  may cause strain gauge structures  24  to become stretched. For example, strain gauge structures  24  may have a lateral dimension L when resting in an unflexed (planar) configuration of the type shown in  FIG. 2 . Following the application of pressure to structure  22  to flex strain gauge structures  24  into the flexed configuration of  FIG. 3 , the magnitude of lateral dimension L may increase to L′ (i.e., L′&gt;L). This elongation in lateral dimension L (e.g., length) and associated reductions in the transverse lateral size of structures  24  (i.e., reductions in width) may increase the resistance of structures  24  when measured along their length. Strain gauge  16  may be used to measure strain produced by the pressure of external object  26  by measuring the amount that the resistance of structures  24  changes as a function of applied force from external object  26 . 
     To increase the amount of measurable change in the resistance of structures  24  that is produced for a given applied pressure from external object  26  (i.e., for a given amount of flex in structure  22 ), one or more variable resistors in structures  24  may be provided with a meandering path, as illustrated by the portion of structures  24  that is shown in the bottom view of  FIG. 4 . Other shapes may be used for structures  24  if desired. Layouts such as the layout of structures  24  of  FIG. 4 , which increase the effective length of structures  24  while retaining a compact (e.g., rectangular) area for structures  24 , are merely illustrative. 
       FIG. 5  is a schematic diagram of illustrative circuitry of the type that may be used in electronic device  10  to gather and process stain gauge signals. As shown in  FIG. 5 , electronic device  10  may include control circuitry such as storage and processing circuitry  32 . Storage and processing circuitry  32  may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in storage and processing circuitry  32  may be used to control the operation of device  10 . This processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio codec chips, application specific integrated circuits, display driver integrated circuits, etc. 
     Storage and processing circuitry  32  may be used to run software on device  10  such as internet browsing applications, voice-over-internet-protocol (VOIP) telephone call applications, email applications, media playback applications, operating system functions, etc. The software may be used to implement control operations such as image acquisition operations using a camera, ambient light measurements using an ambient light sensors, proximity sensor measurements using a proximity sensor, information display functions implemented using status indicators such as light-emitting-diode status indicators, touch event measurements using a touch sensor, functions associated with displaying information on multiple (e.g., layered) displays, operations associated with performing wireless communications functions, operations associated with gathering and producing audio signals, operations associated with gathering and processing button press event data, strain gauge data collection and processing functions, operations associated with responding to strain gauge information, and other functions in device  10 . 
     Input-output circuitry  30  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output circuitry  30  may include sensors such as strain gauges (e.g., strain gauge  16 ), ambient light sensors, light-based and capacitive proximity sensors, touch sensors (e.g., light-based touch sensors and/or capacitive touch sensors that are part of a touch screen display or that are implemented using stand-alone touch sensor structures), accelerometers, and other sensors. Input-output circuitry  30  may also include one or more displays such as display  14  ( FIG. 1 ). Display  14  may be a liquid crystal display, an organic light-emitting diode display, an electronic ink display, a plasma display, a display that uses other display technologies, or a display that uses any two or more of these display configurations. Display  14  may include an array of touch sensors (i.e., display  14  may be a touch screen). The touch sensors may be capacitive touch sensors formed from an array of transparent touch sensor electrodes such as indium tin oxide (ITO) electrodes or may be touch sensors formed using other touch technologies (e.g., acoustic touch, pressure-sensitive touch, resistive touch, optical touch, etc.). Input-output circuitry  30  may include other circuits for handling input and output. For example, input-output circuitry  30  may include communications circuitry for supporting wired and wireless communications, buttons, joysticks, click wheels, scrolling wheels, touch pads, key pads, keyboards, cameras, light-emitting diodes and other status indicators, etc. 
     If desired, strain gauge  16  may be used in forming a button or other user input device for electronic device  10 . Strain gauge  16  may include strain gauge structures  24  such as first strain gauge structures  24 A and second strain gauge structures  24 B. Strain gauge structures  24 A and  24 B may be associated with portions (e.g., first and second respective branches) of a bridge circuit such as a Wheatstone bridge. 
     Control circuitry such as differential amplifier  34  and analog-to-digital converter  36  may be used in gathering and processing strain gauge signals from strain gauge structures  24 . For example, differential amplifier  34  may compare strain gauge signals received from first strain gauge structures  24 A (e.g., a first half of a Wheatstone bridge) to signals received from second strain gauge structures  24 B (e.g., a second half of the Wheatstone bridge) and may produce a corresponding output signal that is indicative of the amount of pressure being applied to structure  22  and gauge  16 . 
     The output signal at the output of differential amplifier  34  may be, for example, an analog voltage signal. Analog-to-digital converter circuitry  36  or other suitable processing circuitry may be used to convert the analog output signal at the output of differential amplifier  34  into digital data for subsequent processing by control circuitry  32 . The control circuitry in device  10  may be configured to take any suitable actions in response to detection of strain gauge input (e.g., by responding as if a button was pressed, etc.). 
     Electronic noise can be coupled into strain gauge  16  through external object  26 . When a user places a finger over strain gauge structures  24 , capacitances arise between the user&#39;s body and strain gauge structures  24 . These capacitances may allow noise such as alternating current (AC) noise to be capacitively coupled into strain gauge structures  24 . The use of a differential strain gauge measurement scheme in strain gauge  16  may help reduce or eliminate such capacitively coupled noise, particularly in situations in which strain gauge structures  24  are configured so that the amount of capacitively coupled noise in the strain gauge is balanced equally between the inputs of differential amplifier  34 . In particular, noise can be reduced by configuring the size, shapes, and locations of strain gauge structures such as structures  24 A and  24 B so as to balance the amount of capacitively coupled noise from finger  26  that appears on differential amplifier input terminals  34 A and  34 B. 
     Strain gauge  16  may be based on a bridge circuit such as the bridge circuit of  FIG. 6  (a Wheatstone bridge). As shown in  FIG. 6 , strain gauge  16  may, for example, include a power supply terminal that is configured to receive a power supply voltage Vs+ (e.g., a direct current voltage). Strain gauge  16  may also include a terminal that is configured to receive a power supply voltage Vs−. Power supply voltage Vs+ may be a positive voltage, a ground voltage, or a negative voltage. Power supply voltage Vs− may be a positive voltage, a ground voltage, or a negative voltage. Power supply voltages Vs+ and Vs− are different, so that a voltage drop equal to the difference between voltage Vs+ and Vs− is applied across the Vs+ and Vs− terminals. 
     Resistors R 1  and R 2  may be coupled in series between terminals Vs+ and Vs−. Resistors R 3  and R 4  may likewise be coupled in series between terminals Vs+ and Vs− (i.e., in parallel with resistors R 1 /R 2 ). Resistors R 1  and R 2  (the left-hand branch of the bridge circuit, shown as structures  24 A in  FIG. 5 ) may form a voltage divider that gives rise to a voltage VA at terminal  34 A (i.e., the node interposed between resistors R 1  and R 2 ). Resistors R 3  and R 4  (the right-hand branch of the bridge circuit, shown as structures  24 B in  FIG. 5 ) may form a voltage divider that gives rise to a voltage VB at terminal  34 B. 
     With one suitable configuration, the nominal resistances of R 1 , R 2 , R 3 , and R 4  in the bridge circuit are identical, so that voltage VA and VB are the same. Other resistance values may be used for R 1 , R 2 , R 3 , and R 4  if desired. The use of nominally equal resistances is merely illustrative. 
     One or more of resistances R 1 , R 2 , R 3 , and R 4  may be implemented using variable resistor structures of the type described in connection with  FIGS. 2, 3, and 4 , whereas the remaining resistors may be implemented using fixed resistors (i.e., resistor structures whose resistance does not change significantly as a function of applied pressure from finger  26 ). By measuring changes in the difference signal (VA-VB) across the inputs of differential amplifier  34 , differential amplifier  34  can produce an output signal that is proportional to the amount of pressure applied with finger  26 . 
     Illustrative configurations of the type that may be used for implementing differential strain gauge circuitry for strain gauge  16  are shown in  FIGS. 7, 8, and 9 . Other types of circuits may be used if desired. The examples of  FIGS. 7, 8, and 9  are merely illustrative. 
     In the illustrative arrangement of  FIG. 7 , strain gauge  16  has been configured so that resistors R 1 , R 2 , and R 4  are fixed and so that resistor R 3  is variable. With this type of arrangement, resistor R 3  may be implemented using a variable resistor strain gauge element such as strain gauge structures  24  of  FIGS. 2, 3, and 4  (i.e., a strain gauge structure formed from a meandering patterned conductive line on the underside of structure  22  having a rectangular outline or other suitable layout). When a user presses on strain gauge  16 , structure  22  will flex and the magnitude of the resistance exhibited by resistor R 3  will increase, while the resistances of resistors R 1 , R 2 , and R 4  will remain fixed. The resulting drop of voltage VA relative to voltage VB can be measured by differential amplifier  34  and analog-to-digital converter  36  and a corresponding digital strain gauge signal may be provided to control circuitry  32  for further processing. 
     In the illustrative arrangement of  FIG. 8 , strain gauge  16  has been configured so that resistors R 1  and R 4  are fixed and so that resistors R 2  and R 3  are variable. With this type of arrangement, resistors R 2  and R 3  may each be implemented using a variable resistor strain gauge element such as strain gauge structures  24  of  FIGS. 2, 3, and 4  (i.e., a strain gauge structure formed from meandering patterned conductive lines on the underside of structure  22  having a rectangular outline or other suitable layout). When a user presses on strain gauge  16 , structure  22  will flex. The flexing of structure  22  will cause the magnitude of the resistance exhibited by resistor R 3  to change (e.g., to increase) and, because resistors R 2  and R 3  are preferably formed adjacent to each other on structure  22 , will also cause the magnitude of resistance R 2  to change (e.g., to increase). The resistances of resistors R 1  and R 4  will remain fixed. In this type of configuration, the pressure on the strain gauge will cause VA to drop and VB to rise. The resulting drop of voltage VA relative to voltage VB can be measured by differential amplifier  34  and analog-to-digital converter  36  for processing by control circuitry  32 . 
     If desired, other types of configurations may be used. For example, if one of the variable resistor structures is implemented using a structure that exhibits an increase in resistance with pressure while the other variable resistor structure exhibits a decrease of resistance with pressure, the variable resistor can be located on R 1  and R 3  (or R 2  and R 4 ), rather than on R 2  and R 3  as in the illustrative example of  FIG. 8 . 
     In the illustrative arrangement of  FIG. 9 , strain gauge  16  has been configured so that resistors R 1  and R 4  exhibit decreases in resistance with increasing pressure, whereas resistors R 1  and R 2  exhibit increases in resistance with increasing pressure. When a user presses on a strain gauge that has been implemented using this type of configuration, structure  22  will flex and the flexing of structure  22  will cause the magnitude of VA to drop relative to the magnitude of VB. Strain can be measured by processing VA and VB using differential amplifier  34 , analog-to-digital converter  36 , and control circuitry  32 . 
     To balance noise effects on inputs  34 A and  34 B of differential amplifier  34  and thereby improve the accuracy of strain gauge  16 , it may be desirable to balance the size, shape, and location of strain gauge structures in the bridge circuitry of strain gauge  16 . For example, it may be desirable to form variable resistor elements from variable resistor line shapes that are of the same size (e.g., that have the same rectangular outline size and shape), that are located adjacent to each other or that are located on top of each other in an overlapping fashion, and/or that have the same line shapes and sizes (e.g., the same linewidths and lengths). Consider, as an example, the arrangement shown in  FIG. 10 . As shown in  FIG. 10 , a user may be applying pressure from finger  26  or other external object to strain gauge  16  (e.g., to first strain gauge structures  24 A and second strain gauge structures  24 B). Noise at the output of differential amplifier  34  may be suppressed by ensuring that capacitive noise coupling from finger  26  to structures  24 A (e.g., one or more resistors in a first branch of a bridge circuit) and  24 B (e.g., one or more resistors in a second branch of the bridge circuit) is equalized (see, e.g.,  FIG. 6 ). 
     With one suitable arrangement (e.g., an arrangement of the type shown in  FIG. 8 ), structures  24 A and  24 B may include variable resistors such as variable resistors R 2  and R 3 , respectively. The size, shape, and location of resistors R 2  and R 3  may be made substantially similar to help equalize capacitive noise coupling. (Capacitive coupling to resistors R 1  and R 4  may likewise be equalized and/or may be minimized by forming resistors R 1  and R 4  from components that are not significantly affected by capacitive coupling from finger  26 ). With this type of arrangement, any noise such as AC noise that is produced by finger  26  on terminal  34 A will also tend to be produced by finger  26  on terminal  34 B. The resulting output of differential amplifier  34  (i.e., the signal on terminal  34 A minus the signal on terminal  34 B) will therefore be relatively unaffected by noise from finger  26 . 
     Bridge circuits such as the circuits of  FIGS. 7 and 9  may likewise be configured so that capacitive coupling to structures  24 A (i.e., structures that affect the value of voltage VA) is comparable to capacitive coupling to structures  24 B (i.e., structures that affect the value of voltage VB). Examples of configurations that may be used to equalize noise coupling to terminals  34 A and  34 B include forming first resistor structures (e.g., variable resistor structures and/or fixed resistor structures) such as those that affect the voltage on terminal  34 A and second resistor structures (e.g., variable resistor structures and/or fixed resistor structures) such as those that affect the voltage on terminal  34 B from meandering paths or other patterns that have the same layout area, from meandering paths or other layouts that overlap (e.g., so that variable resistor R 2  and variable resistor R 3  of an arrangement of the type shown in  FIG. 8  substantially overlap on the underside of structure  22 ), from patterns in which a pair of variable resistors run parallel to each other, or from other structures that tend to be affected similarly due to noise in response to the presence of finger  26  or other external object. 
     If desired, structures  24 A and  24 B may be provided with configurations that are specifically designed to equalize the amount of noise between these structures. For example, structures  24 A may include structures for forming a variable resistor, whereas structures  24 B may contain exclusively or primarily conductive structures that do not change resistance as a function of applied force (as an example). The variable resistor in structures  24 A may, by the nature of its shape, layout, and materials, be subject to picking up a given amount of noise when exposed to an external object during operation. To equalize noise pickup between structures  24 A and  24 B in this type of scenario, the conductive structures in structures  24 B may be configured to have a layout, shape, and material composition that causes structures  24 B to pick up the same amount of noise as structures  24 A. This approach can be used to reduce noise in force sensors with one variable resistor, with two variable resistors, with more than two variable resistors, with other types of force sensing elements (e.g., variable capacitors, variable inductors, etc.). 
     By configuring the structures of strain gauge  16  so that noise signals at the output of the differential amplifier are suppressed, low noise strain gauge output can be provided without needing to resort to filtering schemes with long integration times that might make buttons and other structures formed from the stain gauge  16  exhibit undesirably slow response times. 
       FIG. 11  is a diagram showing how noise coupling on strain gauge elements for the first and second arms of a differential strain gauge may be balanced using elements of equal size. In the example of  FIG. 11 , element  24 A may be formed from a meandering path of conductive material (e.g., material used to implement a variable resistor such as resistor R 2  of  FIG. 8 ), whereas element  24 B may be formed from a meandering path of conductive material with the same shape and size as element  24 A (e.g., material used to implement a variable resistor such as resistor R 3  of  FIG. 8 ). Resistors R 1  and R 2  may be implemented using surface mount technology (SMT) parts, circuitry in an integrated circuit associated with differential amplifier  34 , or other components (as examples). In this type of arrangement, most noise coupling will occur through interactions with the meandering path variable resistors of elements  24 A and  24 B. The use of the same layout and adjacent locations for elements  24 A and  24 B of  FIG. 11  configures elements  24 A and  24 B to balance how much noise is capacitively coupled to the positive terminal of the differential amplifier from the finger of a user with how much noise is capacitively coupled to the negative terminal of the differential amplifier from the finger to reduce noise at an output of differential amplifier  34 . 
     Another way in which noise coupling to variable resistors in strain gauge structures  24 A and  24 B can be balanced to balance noise at the positive and negative differential amplifier terminals is shown in  FIG. 12 . In the example of  FIG. 12 , variable resistors in the first and second arms (e.g., resistors R 2  and R 3  of  FIG. 8  or other suitable strain gauge structures) have been formed from meandering path elements that run parallel to each other. In this example, resistors R 1  and R 4  may be implemented using SMT components or other circuitry that are not substantially capacitively coupled to the user&#39;s finger. Because the path length for the structures that form the R 2  and R 3  variable resistors is equal and the structures are otherwise configured to have equal size and because the location of the variable resistors is nearly identical, noise coupling to the positive and negative inputs to differential amplifier  34  will be equalized. 
       FIG. 13  is a diagram showing how strain gauge elements for the first and second arms of a differential strain gauge may be balanced using structures that overlap each other. In particular, a first variable resistor (e.g., resistor R 2  of  FIG. 8 ) may be implemented using lower structures  24 B, whereas a second variable resistor (e.g., resistor R 3  of  FIG. 8 ) may be implemented using upper structures  24 A. In this example, resistors R 1  and R 4  may be implemented using SMT components or other circuitry that are not substantially capacitively coupled to the user&#39;s finger. As shown in  FIG. 13 , structures  24 A and  24 B may overlap and may have equal size and shape to equalize noise coupling to the positive and negative inputs of differential amplifier  34 . 
     The shapes of strain gauge structures  24 A and  24 B need not be identical. As shown in  FIG. 14 , for example, structures  24 A (e.g., structures for implementing variable resistor R 2  of  FIG. 8 ) may have a U-shape and structures  24 B (e.g., structures for implementing variable resistor R 3  of  FIG. 8 ) may have a different shape such as a meandering path shape. To equalize noise coupling to the positive and negative inputs of differential amplifier  34 , the sizes of elements  24 A and  24 B may be equalized, even though the shapes of elements  24 A and  24 B are different (as an example). 
     If desired, additional pads of metal or other conductive material may be selectively added to structures  24 A or  24 B to help equalize noise coupling to the positive and negative inputs of differential amplifier  34 . As an example, an additional coupling pad that does not serve as an active portion of a variable resistor may be added to structures  24 A or  24 B to increase noise coupling on one side of the strain gauge sufficiently to equalize noise coupling to the positive and negative inputs of differential amplifier  34 . This type of arrangement is shown in  FIGS. 15 and 16 . 
     In the example of  FIG. 15 , structure  24 A- 1  may be a meandering path variable resistor (e.g., resistor R 2  of  FIG. 8 ) and structure  24 B may be a meandering path variable resistor (e.g., resistor R 3  of  FIG. 8 ). Resistors R 1  and R 4  may be formed from SMT parts (or other circuitry that does not couple significantly to the user&#39;s finger compared to resistors R 2  and R 3 ) or may be formed from structures that couple equally to the user&#39;s finger. Coupling pad  24 A- 2  may be added to structures  24 A to help balance the area consumed by structures  24 A and  24 B and thereby equalize noise coupling to the positive and negative inputs of differential amplifier  34 . In the  FIG. 16  example, which may correspond to a strain gauge such as strain gauge  16  of  FIG. 7 , variable resistor R 3  has been implemented using meandering path structure  24 B. Resistors R 1 , R 2 , and R 4  may be implemented as SMT parts (as an example). Coupling pad  24 A′ has been added to structures  24  (i.e., to node VB) to help balance the area consumed by structures  24 A and  24 B and thereby equalize noise coupling to the positive and negative inputs of differential amplifier  34 . 
     In general, any suitable strain gauge design (e.g., any of the designs of  FIGS. 7, 8, and 9  or other designs) may be provided with structures that balance noise coupling to the positive and negative inputs of differential amplifier  34  to reduce noise at the output of the differential amplifier. The examples of  FIGS. 11, 12, 13, 14, 15, and 16  are merely illustrative. 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.

Metadata:
Filing Date: 20170110
Publication Date: 20171017
Grant Date: 20171017
Priority Date: 20111216
Inventors: YANG BINGRUI
GRUNTHANER MARTIN P.
HOTELLING STEVEN P.
Assignee: APPLE INC
CPC Classifications: [{"code": "H04M1/72583", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/0266", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/7258", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/045", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04102", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L1/2262", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/047", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04M1/72469", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/72466", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/04182", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F2203/04102", "inventive": false, "first": false, "tree": "[]"}, {"code": "H03K17/9625", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/045", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03K17/9625", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/047", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/2262", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/22", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/0266", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 47326388