PATENT DOCUMENT

Publication Number: US-10296123-B2
Application Number: US-201514716659-A
Country: US
Kind Code: B2

Title: Reducing noise in a force signal in an electronic device

Abstract:
A force sensing device can be included in a display. Noise produced by various sources can be injected into the force signals produced by the force sensing device. Example noise sources include, but are not limited to, the display, Johnson or Thermal noise from the force sensing device, system noise, and magnetically-coupled or background noise produced by ambient light sources. A sampling scheme that includes one or more noise cancelling techniques can be employed to reduce the amount of noise in the force signals.

Claims:
What is claimed is: 
     
       1. A method of operating a force sensing device incorporated with a display of an electronic device, the method comprising:
 dividing a scanning period for a frame of the display into a plurality of sub scan time periods; and 
 within each sub scan time period, sampling force signals only during a scan period and not sampling force signals during a delay period, wherein the scan and delay periods are offset from one another within a sub scan time period: wherein: 
 a time period between the scan periods in only every other sub scan time period is set to a value that reduces background noise in the force signals. 
 
     
     
       2. The method as in  claim 1 , wherein a signal instability produced by a VCOM signal transitioning between high and low signal levels occurs only during the delay period. 
     
     
       3. The method as in  claim 1 , wherein a signal instability produced by a VCOM signal transitioning between high and low signal levels partially overlaps with the scan period. 
     
     
       4. The method as in  claim 1 , wherein the background noise is produced by A/C power. 
     
     
       5. The method as in  claim 1 , further comprising switching reference voltages received by one or more strain sensing elements in the force sensing device between subscan time periods. 
     
     
       6. An electronic device, comprising: a display stack for a display, comprising:
 a cover glass; and 
 a strain sensing structure positioned below the cover glass, the strain sensing structure comprising a substrate, a first set of strain sensitive films positioned on a first surface of the substrate, and a second set of strain sensitive films positioned on a second surface of the substrate and aligned vertically with the first set of strain sensitive films, wherein a strain sensitive film in the first set and a vertically aligned strain sensing film in the second set together form a strain sensing element; wherein a strain sensitive film in the first set and a vertically aligned strain sensing film in the second set together from a strain sensing element; 
 sense circuitry operably connected to each strain sensing element; and 
 a controller operably connected to the sense circuitry and configured to cause force signals to be sampled from one or more strain sensing elements multiple times during a frame scanning period of the display, wherein the force signals are sampled only during multiple scan periods in the frame scanning period and the scan periods are offset from one another by a delay period: wherein: 
 
       a time period between only every other scan period is set to a value that reduces background noise in the force signals. 
     
     
       7. The electronic device as in  claim 6 , wherein the background noise is produced by A/C power. 
     
     
       8. The electronic device as in  claim 6 , wherein the display stack further comprises a display layer and a VCOM buffer layer and a signal instability produced by a VCOM signal transitioning between high and low signal levels occurs only during the delay periods. 
     
     
       9. The electronic device as in  claim 6 , wherein the controller is configured to switch reference voltages received by each strain sensing element in the strain sensing structure between scan periods. 
     
     
       10. The electronic device as in  claim 6 , wherein the sense circuitry comprises a sigma delta analog-to-digital converter. 
     
     
       11. The electronic device as in  claim 10 , wherein a size of a step for one analog-to-digital conversion is selected to produce sufficient attenuation at a display line refresh noise frequency. 
     
     
       12. A method of operating a force sensing device incorporated with a display of an electronic device, wherein the force sensing device comprises a strain sensing element, the method comprising:
 dividing a scanning period for a frame of the display into a plurality of sub scan time periods: 
 within each sub scan time period, sampling force signals only during a scan period and not sampling force signals during a delay period, wherein the scan and delay periods are offset from one another within each sub scan time period: and 
 flipping a first reference voltage and a second reference voltage between first and second nodes of the strain sensing element when a force signal not sampled from the strain sensing element: wherein: 
 a time period between the scan periods in only every other sub scan time period Is set to a value that reduces background noise in the force signals. 
 
     
     
       13. The method as in  claim 12 , wherein the reference voltages are flipped during the delay periods. 
     
     
       14. The method as in  claim 12 , wherein a signal instability produced by a VCOM signal in the display transitioning between high and low signal levels occurs only during the delay period. 
     
     
       15. The method as in  claim 12 , wherein the background noise is produced by A/C power. 
     
     
       16. A method of operating a force sensing device incorporated with a display of an electronic device, wherein the force sensing device comprises a strain sensing element, the method comprising:
 sampling a force signal from the strain sensing element during a scan period in at least one sub scan time period, wherein a plurality of sub scan time periods occur during a scanning period for a frame of the display and the scan period is offset from a signal instability produced by a VCOM signal in the display transitioning between two signal levels: wherein: 
 a time period between the scan periods in only every other sub scan time period is set to a value that reduces background noise in the force signals. 
 
     
     
       17. The method as in  claim 16 , further comprising flipping a first reference voltage and a second reference voltage between first and second nodes of the strain sensing element when the force signal not sampled from the strain sensitive element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 62/129,634, filed Mar. 6, 2015, entitled “Reducing Noise in a Force Signal in an Electronic Device,” the entirety of which is incorporated herein by reference as if fully disclosed herein. 
    
    
     TECHNICAL FIELD 
     Embodiments described herein generally relate to force sensing, and in particular to techniques for reducing noise in a force signal produced by a strain sensing element in an electronic device. 
     BACKGROUND 
     Many electronic and input devices include a touch-sensitive surface for receiving user inputs. Devices such as smart telephones, tablet computing devices, laptop computers, track pads, wearable communication and health devices, navigation devices, and kiosks can include a touch-sensitive surface. In some cases, the touch sensitive surface is integrated with a display to form a touch-screen or touch-sensitive display. 
     The touch-sensitive surface may detect and relay the location of one or more user touches, which may be interpreted by the electronic device as a command or a gesture. In one example, the touch input may be used to interact with a graphical user interface presented on the display of the device. In another example, the touch input may be relayed to an application program operating on a computer system to affect changes to the application program. 
     Touch sensitive surfaces, however, are limited to providing only the location of one or more touch events. Moreover, touch, like many present inputs for computing devices, is binary. The touch is either present or it is not. Binary inputs are inherently limited insofar as they can only occupy two states (present or absent, on or off, and so on). In many examples, it may be advantageous to also detect and measure the force of a touch that is applied to a surface. In addition, if the force can be measured across a continuum of values, it can function as a non-binary input. Further, incorporating a touch sensing device and a force sensing device with the display of an electronic device may provide an enhanced user input for controlling an application or function of the electronic device as compared to using a touch sensor alone. 
     One challenge with incorporating a force sensing device into the display of an electronic device is that signals associated with the display and other components in the electronic device can introduce noise into the force signals produced by the force sensing device. The noise can cause errors in the force measurements. Additionally, the noise produced by the display and other components can overwhelm the force signals in that the magnitude of the noise can be much greater than the magnitude of the force signals, making it difficult to discern the force signals from the noise. 
     SUMMARY 
     A force sensing device can be incorporated into a display stack in an electronic device. The force sensing device can include a first set of individual strain sensitive films formed on a first surface of a substrate and a second set of individual strain sensitive films formed on a second surface of a substrate. Each strain sensitive film in the first set is aligned with a respective strain sensitive film in the second set. Two aligned strain sensitive films in the first and second sets together form a strain sensing element. 
     In certain situations, noise can be injected into a force signal when the force signal is sampled from one or more strain sensing elements. The noise can be generated by various sources within and outside of the electronic device. Example noise sources include, but are not limited to, the display, Johnson noise from the force sensing device, and magnetically-coupled or background noise produced by ambient AC power (operating at 60 Hz in the United States and 50 Hz in Europe), and noise due to a battery charging circuit. Embodiments of a sampling scheme are disclosed herein that can be employed to reduce or cancel the noise produced by one or more sources. 
     A single scanning period for a frame of the display may be divided into multiple subscan time periods, and the scanning operation for the frame is divided into multiple subscan operations. In some embodiments, a subscan operation occurs in each subscan time period. Thus, the subscan operations are repeated several times within the frame scanning time period. The subscan operations are performed only during a portion of a subscan time period. A delay period occurs between successive subscan operations. Thus, each subscan time period includes a delay period and a subscan time period. 
     In embodiments where the display includes a VCOM buffer layer, a VCOM signal may transition from a first level to a second level for touch sensing functions and transition from the second level to the first level for display functions. In some embodiments, signal instabilities may occur when the VCOM signal transitions between the touch sensing and the display functions (“touch-to-display handoff noise”). Some or all of the touch-to-display handoff noise may be injected into the force signals if the force signals are sampled during this time. Thus, in some embodiments, a delay period occurs during the time period the touch-to-display handoff noise is produced. The delay period can provide a sufficient settling time period prior to sampling the force signals. The delay time period is tunable or customizable in that any suitable time period can be used. The delay time period, which influences the start and stop times for the subscan operation in the subscan time period, may reduce or eliminate the amount of touch-to-display handoff noise that is injected into the force signals. In some embodiments, a delay time period can cause a subscan operation to be completely offset from the touch-to-display handoff noise. In other embodiments, a delay time period may reduce the amount of time a subscan operation overlaps the time in which the touch-to-display handoff noise is produced. In some embodiments, a signal instability produced by a VCOM signal transitioning between high and low signal levels can partially overlap with a scan period. 
     Additionally or alternatively, the time periods between every other subscan operation (“a subscan pair”) can be selected to reduce or eliminate background noise. Background noise can be magnetically coupled noise from ambient light sources. For example, light sources in some countries, such as in the United States, operate at 60 Hz while in other countries (e.g., Europe) the light sources operate at 50 Hz. Thus, in some embodiments, a time period between subscan pairs can be selected to reduce or cancel the amount of background noise in the force signals. 
     Additionally or alternatively, the sampling scheme can reduce the Johnson noise produced by the strain sensing elements. The time period between successive subscan operations may be selected to allow the noise to continue integrating using one or more analog filtering elements. A capacitor is one non-limiting example of an analog filtering element, but other types of analog filtering elements can be used. The analog filtering element(s) may continue to operate when the analog signals are not being converted to digital signals. The noise can be averaged when multiple subscan operations are performed with delay periods between the subscan operations. 
     In some embodiments, system noise may be injected into the force signals. Circuits and components, along with subsystems, may be directly or indirectly involved in creating the system noise. For example, temperature changes due to dissipation in a battery or microprocessor may induce a Seebeck voltage offset in the sensor circuit. The magnetic field due to the current driving an audio device may contribute an electromotive force (EMF) in a sensor circuit. In another example, the magnetic field due to proximity of a moving magnet (such as that in a pair of headphones) may induce a changing EMF in a sensor circuit. In some embodiments, bias flipping can be used to cancel or reduce the system noise. Each strain sensing element receives a first reference voltage at a first node and a second reference voltage at a second node. The first and second reference voltages can alternately switch between the first and second nodes to reduce or cancel the amount of system noise injected into the force signals. In one non-limiting example, the first and second reference voltages flip in between the subscan operations. In another non-limiting example, the first and second reference voltages flip in between the full scan operations. Other embodiments can perform bias flipping in another suitable pattern or arrangement to cancel or reduce the system noise. 
     Additionally or alternatively, sense circuitry operably connected to the strain sensing elements can include a filter that can be configured to reduce or eliminate display line refresh noise. Since the frequencies of the display line refresh noise can be determined, a transfer function is selected for the filter to reduce or eliminate the display line refresh noise. In one non-limiting example, the sense circuitry can include an analog-to-digital converter with a windowing or decimation filter. The windowing or decimation filter can be configured to filter out the display line refresh noise or other noise sources that show a concentration of noise energy in one or multiple narrow band frequency bins or other types of tonality in the noise spectrum. The decimation filter attenuates high frequency noise, so a size of a step for one analog-to-digital conversion may be selected to produce sufficient attenuation at the frequency of the display line refresh noise. 
    
    
     
       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  is a conceptual view of a display screen that can be used to perform multiple functions; 
         FIG. 2  depicts one example of an electronic device that can be configured to perform multiple functions with respect to a display; 
         FIG. 3  is a block diagram of a system that includes a display, a force sensing device, and a touch sensing device; 
         FIG. 4  illustrates a simplified plan view of an example strain-sensitive structure including a grid of optically transparent strain-sensitive films; 
         FIG. 5  depicts a plan view of one example of an optically transparent serpentine strain-sensitive film which may be used in the example strain-sensitive structure depicted in  FIG. 4 ; 
         FIG. 6  is a cross-sectional view of a portion of the display  204  taken along line  6 - 6  in  FIG. 2 ; 
         FIG. 7  is a simplified cross-sectional view of the strain sensing structure  610  responding to force; 
         FIG. 8  is a simplified schematic diagram of sense circuitry operably connected to a strain sensing element; and 
         FIG. 9  illustrates one example of a timing diagram for a sampling scheme that is suitable for use with the strain-sensitive structure shown in  FIG. 6 ; 
         FIG. 10  is a simplified block diagram of the strain sensing element  800  operatively connected to a portion of the sense circuitry  812  shown in  FIG. 8 ; and 
         FIG. 11  is a flowchart of a method of cancelling noise in force signals produced by a strain sensitive structure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments described herein provide an electronic device that includes a display and multiple devices that each use or share at least a portion of the display area. By way of example only, the multiple devices can include a touch sensing device and a force sensing device. The touch and force sensing devices can each use at least a portion of the top surface of the display screen as an input region. 
     In some embodiments, noise produced by various sources can be injected into the force signals produced by the force sensing device. Example noise sources include, but are not limited to, the display, Johnson or Thermal noise from the force sensing device, and magnetically-coupled or background noise produced by ambient light sources (e.g., light sources operating at 60 Hz in the United States and 50 Hz in Europe). Various techniques are disclosed herein that reduce or cancel the noise in the force signals. A sampling scheme that includes one or more noise cancelling techniques can be employed to reduce or remove noise from the force signals. All of the noise cancelling techniques can be used together when sampling force measurements, or individual noise cancelling techniques can be used individually or in various combinations. 
     Directional terminology, such as “top”, “bottom”, “front”, “back”, “leading”, “trailing”, etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments described herein can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration only and is in no way limiting. When used in conjunction with layers of a display or device, the directional terminology is intended to be construed broadly, and therefore should not be interpreted to preclude the presence of one or more intervening layers or other intervening features or elements. Thus, a given layer that is described as being formed, positioned, disposed on or over another layer, or that is described as being formed, positioned, disposed below or under another layer may be separated from the latter layer by one or more additional layers or elements. 
     Referring now to  FIG. 1 , there is shown a conceptual view of a display that can be used to perform multiple functions. The functions can include a display function  100 , a force sensing function  102 , and a touch sensing function  104 . These functions can be performed in conjunction with the display  106 . In other words, a user can interact with an image displayed on the display  106  with one or more touches, an applied force, or both touch and force. For example, a game that is displayed on the display  106  can receive touch inputs from a user. As another example, an application displayed on the display  106  can perform one function at one rate of speed when a user applies a small amount of force to the display and perform the function at a faster rate of speed when the user applies a greater amount of force to the display  106 . 
     The touch sensing and force sensing functions can each use or share some or all of the display area. For example, in one embodiment, a user can interact with a displayed image by touching and/or by applying a force at an appropriate position on the display, with the appropriate position located anywhere on the display. In another embodiment, the display function  100  and the touch sensing function  104  can use the entire display  106  while the force sensing function  102  involves a portion of the display  106 . Thus, each function can use some or all of the display  106  when in operation. The arrangement of the functions in  FIG. 1  is for illustrative purposes only, and does not correspond to any layers or devices in the display or in an electronic device. Additionally, the arrangement of the functions does not correspond to the amount of area on the display used by each function. 
       FIG. 2  depicts one example of an electronic device that can be configured to perform multiple functions with respect to a display. In the illustrated embodiment, the electronic device  200  is implemented as a smart telephone. Other embodiments can implement the electronic device differently. For example, an electronic device can be a laptop computer, a tablet computing device, a wearable computing device, a digital music player, a display input device, a kiosk, a remote control device, a television, and other types of electronic devices that include a display. 
     The electronic device  200  includes an enclosure  202  surrounding a display  204  and one or more input/output devices  206  (shown as button  206 ). The enclosure  202  can form an outer surface or partial outer surface and protective case for the internal components of the electronic device  200 , and may at least partially surround the display  204 . The enclosure  202  can be formed of one or more components operably connected together, such as a front piece and a back piece. Alternatively, the enclosure  202  can be formed of a single piece operably connected to the display  204 . 
     The display  204  can be implemented with any suitable display, including, but not limited to, a multi-touch sensing touchscreen device that uses liquid crystal display (LCD) technology, light emitting diode (LED) technology, organic light-emitting display (OLED) technology, or organic electro luminescence (OEL) technology. 
     In some embodiments, the button  206  can take the form of a home button, which may be a mechanical button, a soft button (e.g., a button that does not physically move but still accepts inputs), an icon or image on a display, and so on. Further, in some embodiments, the button  206  can be integrated as part of a cover glass of the electronic device. Although not shown in  FIG. 2 , the electronic device  200  can include other types of input/output devices, such as a microphone, a speaker, a camera, and one or more ports, such as a network communication port and/or a power cord port. 
     Embodiments described herein perform a display operation, a touch sensing operation, and a force sensing operation in a display area or in one or more portions of a display area.  FIG. 3  is an example block diagram of a system that includes a display  320 , a force sensing device  308 , and a touch sensing device  310 . A processing device  302  is operably connected to a storage device  304 , a display controller  306 , the force sensing device  308 , and the touch sensing device  310 . Image data is received by the processing device  302  on signal line  312  and stored in the storage device  304 . The processing device  302  can be implemented with one or more suitable data processing devices, examples of which include a microprocessor, an application-specific integrated circuit (ASIC), and a central processing unit (CPU). The storage device  304  can be configured as one or more memories, including, but not limited to, RAM, ROM, flash memory, and removable memory, or combinations thereof. 
     The display controller  306  can include a timing controller  314  that generates timing and control signals for the display  320 , the force sensing device  308 , and the touch sensing device  310 . For example, the timing controller  314  and/or the display controller  306  can produce timing and control signals that control the sampling time periods and non-sampling time periods of the force and touch signals. The display controller  306  can be any suitable hardware, software, firmware, or combination thereof adapted to translate the image data into control signals for driving the pixels  318  of the display  320 . The display controller  306  can include other suitable components, such as a processing device and/or a storage device. 
     The processing device  302  receives force signals from the force sensing device  308  on signal line  322 . The processing device  302  determines an amount of force, or a change in force, applied to an input region of the force sensing device  308  based on at least one force signal. Additionally, the processing device  302  receives the touch signals from the touch sensing device  310  on signal line  324 . The processing device  302  determines one or more touch locations on an input region of the touch sensing device  310  based on at least one touch signal. In some embodiments, the processing device  302  produces the timing and control signals that control the sampling time periods and non-sampling time periods of the force and touch signals. 
     It should be noted that  FIGS. 2 and 3  are illustrative only. In other examples, an electronic device may include different, fewer, or more components than those shown in  FIGS. 2 and 3 . 
     The touch sensing device and the force sensing device can employ any suitable sensing technology. By way of example only, a force sensing device and a touch sensing device can use capacitive sensing technology, resistive sensing technology, piezoelectric or piezoresistive sensing technology, magnetic technology, optical technology, inductive technology, and ultrasonic sensing technology. In the embodiments described herein, the force sensing device is implemented as a force sensitive film that produces a signal or a change in a signal in response to strain. The signal is used to determine or estimate an amount of force applied to an input region. The one or more strain-sensitive films are configured as strain gauges that are formed with a piezoresistive material. Also in the embodiments described herein, the touch sensing device is implemented as a capacitive touch sensing device that determines a location of one or more touches applied to an input region through capacitance changes in one or more capacitive sensing elements. Other embodiments can use a different type of a force sensing device and/or touch sensing device, including, but not limited to, resistive, ultrasonic, thermal, capacitive, or piezoelectric devices. 
       FIG. 4  depicts a plan view of an example strain-sensitive structure including a grid of optically transparent strain-sensitive films. The strain-sensitive structure  400  includes a substrate  402  with independent strain-sensitive films  404  formed in or on the substrate  402 . The strain-sensitive films  404  are configured to detect strain based on an amount of force applied to an input region. In this example, the substrate  402  may be an optically transparent material, such as polyethylene terephthalate (PET). The strain-sensitive films  404  may be made from transparent conductive materials include, for example, polyethyleneioxythiophene (PEDOT), indium tin oxide (ITO), carbon nanotubes, graphene, silver nanowire, other metallic nanowires, and the like. In certain embodiments, the strain-sensitive films  404  may be selected at least in part on temperature characteristics. For example, the material selected for strain-sensitive films  404  may have a negative temperature coefficient of resistance such that, as temperature increases, the resistance decreases. 
     In this example, strain-sensitive films  404  are formed as an array of rectilinear sensing elements, although other shapes and array patterns can also be used. In many examples, each individual strain-sensitive film  404  may have a selected shape and/or pattern. For example, in certain embodiments, a strain-sensitive film  404  may be deposited in a serpentine pattern, such as the one shown in  FIG. 5 . The strain-sensitive film  404  may include at least two electrodes  500 ,  502  that are configured to be operably connected to sense circuitry. In other cases, a strain-sensitive film  404  may be electrically connected to sense circuitry without the use of electrodes. For example, a strain-sensitive film  404  may be connected to the sense circuitry using conductive traces that are formed as part of the film layer. 
       FIG. 6  depicts a cross-sectional view of the display  204  taken along line  6 - 6  in  FIG. 2 . The cross-sectional view illustrates a display stack  600  for the display  204 . A cover glass  601  is positioned over a front polarizer  602 . The cover glass  601  can be a flexible touchable surface that is made of any suitable material, such as, for example, a glass, a plastic, sapphire, or combinations thereof. The cover glass  601  can act as an input region for a touch sensing device and a force sensing device by receiving touch and force inputs from a user. The user can touch the cover glass  601  with one or more fingers or with another element such as a stylus. 
     An adhesive layer  604  can be disposed between the cover glass  601  and the front polarizer  602 . Any suitable adhesive can be used in adhesive layer, such as, for example, a liquid optically clear adhesive. A display layer  606  can be positioned below the front polarizer  602 . As described previously, the display layer  606  may take a variety of forms, including a liquid crystal display (LCD), a light-emitting diode (LED) display, and an organic light-emitting diode (OLED) display. In some embodiments, the display layer  606  can be formed from glass or have a glass substrate. Embodiments described herein include a multi-touch touchscreen LCD display layer. 
     Additionally, the display layer  606  can include one or more layers. For example, a display layer  606  can include a VCOM buffer layer, a LCD display layer, and a conductive layer disposed over and/or under the display layer. In one embodiment, the conductive layer may comprise an indium tin oxide (ITO) layer. 
     A rear polarizer  608  may be positioned below the display layer  606 , and a strain sensitive structure  610  below the rear polarizer  608 . The strain-sensitive structure  610  includes a substrate  612  having a first set of independent strain-sensitive films  614  on a first surface  616  of the substrate  612  and a second set of independent strain-sensitive films  618  on a second surface  620  of the substrate  612 . In the illustrated embodiment, the first and second surfaces  616 ,  620  are opposing top and bottom surfaces of the substrate  612 , respectively. An adhesive layer  622  may attach the substrate  612  to the rear polarizer  608 . 
     As described earlier, the strain-sensitive films may be formed as an array of rectilinear strain sensing elements. Each strain-sensitive film in the first set of independent strain-sensitive films  614  is aligned vertically with a respective one of the strain-sensitive films in the second set of independent strain-sensitive films  618 . In many embodiments, each individual strain-sensitive film may take a selected shape. For example, in certain embodiments, the strain-sensitive film may be deposited in a serpentine pattern, similar to the serpentine pattern shown in  FIG. 5 . 
     A back light unit  624  can be disposed below the strain sensitive structure  610 . The back light unit  624  may be configured to support one or more portions of the substrate  612  that do not include strain-sensitive films. For example, as shown in  FIG. 6 , the back light unit  624  can support the ends of the substrate  612 . Other embodiments may configure a back light unit differently. 
     The strain-sensitive films are typically connected to sense circuitry  626  through conductive connectors  628 . The sense circuitry  626  is configured to detect changes in an electrical property of each of the strain-sensitive films. In this example, the sense circuitry  626  may be configured to detect changes in the resistance of the strain-sensitive films, which can be used to estimate a force that is applied to the cover glass  601 . In some embodiments, the sense circuitry  626  may also be configured to provide information about the location of a touch based on the relative difference in the change of resistance of the strain-sensitive films  614 ,  618 . 
     For example, as discussed earlier, the strain sensitive films can be configured as strain gauges that are formed with a piezoresistive material. When a force is applied to an input region (e.g., the cover glass  601 ), the planar strain sensitive structure  610  is strained and the resistance of the piezoresistive material changes in proportion to the strain. As shown in  FIG. 7 , the force can cause the strain sensitive structure  610  to bend slightly. The bottom  700  of the strain sensitive structure elongates while the top  702  compresses. The strain gauges measure the elongation or compression of the surface, and these measurements can be correlated to the amount of force applied to the input region. 
     Two vertically aligned strain-sensitive films (e.g.,  630  and  632 ) form a strain sensing element  634 . The sense circuitry  626  may be adapted to determine a difference in an electrical property of each strain sensing element. For example, as described above, a force may be received at the cover glass  601 , which in turn causes the strain sensitive structure  610  to bend. The sense circuitry  626  is configured to detect changes in an electrical property (e.g., resistance) of the one or more strain sensing elements, and these changes are correlated to the amount of force applied to the cover glass  601 . 
     In the illustrated embodiment, a gap  636  exists between the strain sensitive structure  610  and the back light unit  624 . Strain measurements intrinsically measure the force at a point on the top surface  616  of the substrate  612  plus the force from the bottom at that point on the bottom surface  620  of the substrate  612 . When the gap  636  is present, there are no forces on the bottom surface  620 . Thus, the forces on the top surface  616  can be measured independently of the forces on the bottom surface  620 . In alternate embodiments, the strain sensitive structure  610  may be positioned above the display layer when the display stack  600  does not include the gap  636 . 
     Referring now to  FIG. 8 , there is shown a simplified schematic diagram of sense circuitry operably connected to a strain sensing element. The strain sensing element  800  that includes the two-vertically aligned strain-sensitive films can be modeled as two resistors R SENSE  configured as a voltage divider. A reference voltage divider  802  includes two reference resistors R REF . As one example, the strain sensing element  800  and the reference voltage divider  802  may be modeled as a Wheatstone bridge circuit, with the strain sensing element  800  forming a half bridge of the Wheatstone bridge circuit and the reference voltage divider forming the other half bridge of the Wheatstone bridge circuit. Other embodiments can model the strain sensing element and the reference resistors differently. 
     A first reference voltage (V REF   _   TOP ) is received at node  804  and a second reference voltage (V REF   _   BOT ) is received at node  806 . A force signal at node  808  of the strain sensing element  800  and a reference signal at node  810  of the reference voltage divider  802  are received by the sense circuitry  812 . The sense circuitry  812  is configured to detect changes in an electrical property (e.g., resistance) of the strain sensing element  800  based on the differences in the force and reference signals of the two voltage dividers. The changes can be correlated to the amount of force applied to the cover glass  601 . 
       FIG. 9  is one example of a timing diagram for a sampling scheme that is suitable for use with the strain-sensitive structure shown in  FIG. 6 . The sampling scheme can be implemented in a processing device or controller, such as in the display controller  306  shown in  FIG. 3 . The timing generator  314  can produce the timing and control signals that produce the illustrated timing diagram. 
     As described earlier, noise from several different sources can be injected into the force signal received from one or more strain sensing elements when the force signal is sampled. The noise sources include touch-to-display handoff noise, Johnson noise, background or magnetically-coupled noise from ambient light sources, general noise from the system, and display line refresh noise. The timing diagram shown in  FIG. 9  provides a scanning scheme that can reduce or eliminate noise from some or all of these noise sources. Thus, the scanning scheme may be a unified noise cancelling scanning scheme when all of the noise cancelling techniques described below are employed. Other embodiments, however, are not limited to using all of the noise cancelling techniques in an electronic device. One or more of the noise cancelling techniques may be used in other embodiments. 
     In the illustrated embodiment, the time period T 1  (time between T 2  and T 7 ) represents a single scanning period for a frame of the display (e.g., display  204 ). The time period T 1  is divided into multiple subscan time periods T 2  to T 3 , T 3  to T 4 , T 4  to T 5 , T 5  to T 6 , and T 6  to T 7 . Additionally, a scanning operation for the frame is divided into multiple subscan operations  900 . As shown, a subscan operation  900  occurs in each subscan time period. Thus, the subscan operations  900  are repeated several times within the frame time period T 1 . In other embodiments, the subscan operations  900  can occur in select subscan time periods. In some embodiments, the subscan operations can subsequently be accumulated to introduce another level of filtering, as described below. The signals or values obtained during the subscan operations may or may not be weighted when accumulated. 
     Touch-to-Display Handoff Noise 
     The BSYNC signal is a system level synchronization signal. In one embodiment, touch sensing functions occur when the BSYNC signal is high and display functions occur when the BSYNC signal is low. In the illustrated embodiment, a VCOM signal is received by the VCOM plane in the display layer. The VCOM signal may transition from a first level (e.g., a high level) to a second level (e.g., a low level) for touch sensing functions (see  902 ), and transition from the second level to the first level for display functions (see  904 ). In some embodiments, signal instabilities may occur when the VCOM signal transitions between the touch sensing and the display functions (“touch-to-display handoffs”). For example, as shown in  FIG. 9 , noise  906  (“touch-to-display handoff noise”) may be produced during the VCOM signal transitions, and some or all of this noise  906  can be injected into the force signals if the force signals are sampled during this time. Thus, in some embodiments, a delay period  908  may occur before a subscan operation  900 . The delay period  908  can provide a sufficient settling time period prior to sampling the force signals. 
     Each delay time period  908  is tunable or customizable in that any suitable time period can be used. Additionally, the delay time periods  908  can be the same amount of time, or some delay time periods can have a different amount of time compared to other delay time periods. The delay time periods, which influence the start and stop times for the subscan operations  900 , may reduce or eliminate the amount of touch-to-display handoff noise  906  that is injected into the force signals. In some embodiments, a delay time period  908  can cause a subscan operation  900  to be completely offset from the touch-to-display handoff noise  906 . In other embodiments, a delay time period  908  may reduce the amount of time a subscan operation  900  overlaps the time period in which the touch-to-display handoff noise  906  is produced. In other words, the delay time period can reduce the amount of time a subscan operation  900  and the touch-to-display handoff noise  906  occur simultaneously. 
     Background Noise 
     Additionally or alternatively, the time periods between every other subscan operation  900  (“a subscan pair”) can be selected to reduce or eliminate background noise. As described earlier, the background noise can be magnetically coupled noise from ambient light sources. For example, light sources in some countries, such as in the United States operate at 60 Hz while in other countries (e.g., Europe) the light sources operate at 50 Hz. Thus, in some embodiments, the time period t 1  between subscan pairs can be selected to reduce or cancel the amount of background noise that is injected into the force signals. For example, in one embodiment with a 16 Hz frame time period, noise from a 60 Hz light source can be cancelled out when the time period t 1  is 8.33 ms. In other embodiments, other types of noise may be cancelled out with an appropriate amount of time for time period t 1 . For example, the time period t 1  can be set to an appropriate amount of time (e.g., 10 ms) to cancel the noise from 50 Hz light sources. 
     In some embodiments, the time period t 2  between successive subscan operations can assist in noise cancellation. For example, the time period t 1  can be set to cancel noise at one frequency band while the time period t 2  may be set to cancel noise at different second frequency band. In other words, the time periods t 1  and t 2  can be set for multiple frequency domain filter notches. The time periods t 1  and t 2  may be used to implement various kinds of Finite Impulse Response (FIR) filters that can be programmed to attain certain filter characteristics. As one non-limiting example, the time period t 1  can be set to 8.33 ms to cancel out 60 Hz noise and the time period t 2  may be set to 3.5 ms to cancel out 143 Hz noise. 
     Additionally or alternatively, the time periods t 1  and t 2  can be set to avoid system interferences in the time domain. For example, the time period t 1  may be set to not perform a scan operation when disturbances, such as touch-to-display handoff noise occurs. 
     Johnson Noise 
     Additionally, performing a subscan operation  900  only during a portion of a subscan time period may reduce or eliminate the effect of Johnson noise while improving or optimizing circuit power consumption. In certain embodiments, Johnson noise may be the dominant noise source due to the relatively high resistance of the strain sensitive films. In one embodiment, the integration bandwidth of the Johnson noise can be controlled by an analog filtering component, such as the capacitor C B  in  FIG. 8 . Depending on the selection of the delay time periods  908  and a sufficiently large value of capacitance for C B , the Johnson noise may average for a longer period of time than the period of time the sense circuitry is operating (e.g., the ADC  1014  in  FIG. 10 ), which reduces power consumption. In other words, less power is consumed by sampling the force signals only during the multiple subscan operations compared to sampling the force signals for an entire frame scanning period T 1 . The delay period  908  reduces the amount of power consumed during each subscan time period. This technique may be especially useful in a multiplexed ADC system (see e.g., sense circuitry  812  in  FIG. 10 ). Additionally, the capacitor C B  is a passive analog filtering component that averages the sampled force signals over the entire frame scanning period T 1 . 
     In some embodiments, a delay period  908  before each subscan operation  900  may not adversely affect the signal-to-noise ratio (SNR) when the RC time (R SENSE *C B ; see  FIG. 8 ) is of a sufficient length. The noise can be averaged after filtering from the capacitor C B  when multiple subscan operations are performed with delay periods between the subscan operations. 
     Additionally or alternatively, other embodiments can employ different techniques for reducing or eliminating Johnson noise. For example, in one embodiment the value of R SENSE in  FIG. 8  can be decreased. The signals \T REF   _   TOP  and \T REF   _   BOT  may be increased. And the capacitance value for C B  can be increased. One or more of these different techniques can be used in place of bias flipping. Alternatively, one or more of these different techniques can be used in combination with bias flipping. 
     System Noise 
     System noise can be created by circuits and components in the electronic device. As one example, noise from a power supply or coupling noise can produce system noise. Some or all of the system noise may be injected into the force signals when the force signals are sampled. Bias flipping can be used in some embodiments to cancel or reduce the system noise, such as low frequency system noise. As shown in  FIG. 9 , the signal level for V REF   _   TOP  and V REF   _   BOT  can alternately switch from a first level (e.g., high level) to a second level (e.g., low level) during the frame time period T 1 . As described earlier, the signals V REF   _   TOP  and V REF   _   BOT  are received at nodes  804  and  806  in  FIG. 8 . Switching or flipping the signal levels can differentiate out the low frequency voltage noise. 
     The timing of when the bias switching occurs can depend on the type of noise being cancelled. To cancel system noise, the signal levels switch in between subscan operations  900  in the illustrated embodiment. Other embodiments can switch the signal levels differently. For example, the signal levels can switch on a frame-to-frame basis. 
     Additionally or alternatively, bias flipping can be used to reduce or cancel other types of magnetically and electrostatically coupled noise. As one example, the Seebeck effect is noise produced by micro-temperature fluctuations at the contact locations of different types of metals or conductors. Bias flipping can reduce or cancel the noise produced by the Seebeck effect. 
     Referring now to  FIG. 10 , there is shown a simplified block diagram of a strain sensing element  800  operatively connected to a portion of the sense circuitry  812  shown in  FIG. 8 . The force signal at node  808  is received by a multiplexer  1002  on signal line  1004 . The multiplexer  1002  also receives force signals from other strain sensing elements on one or more additional signal lines  1006 . A force signal output from the multiplexer  1002  on signal line  1008  is received by an amplifier circuit  1010 . The reference signal from the reference voltage divider (see  802  in  FIG. 8 ) is received by the amplifier circuit  1010  on signal line  1012 . As one example, the amplifier circuit  1010  may be a differential programmable gain amplifier. The differential programmable gain amplifier amplifies the difference between the force and reference signals received on signal lines  1008  and  1012 , respectively. 
     An output signal from the amplifier circuit  1010  is received by an analog-to-digital converter (ADC)  1014 . The ADC converts the analog output signal to a digital output signal. The digital output signal may then be processed further to correlate the digital signal to an amount of force applied to an input region (e.g., cover glass  601 ). 
     The sense circuitry can be configured as multiple channels with each channel receiving force signals from two or more strain sensing elements. The number of channels may be determined, at least in part, by the number of multiplexers and the number of ADCs that will be included in the system. For example, in one embodiment a system can include eight channels with the sense circuitry including four M:1 multiplexers and eight ADCs. Alternatively, in another embodiment a system may include four channels with the sense circuitry including eight M:1 multiplexers and four ADCs. 
     Display Line Refresh Noise 
     In one embodiment, the ADC  1014  is a sigma delta ADC. The sigma delta ADC can be configured to reduce or eliminate the display line refresh noise. Since the frequencies of the display line refresh noise can be determined, a transfer function may be selected to reduce or eliminate the display line refresh noise. In particular, the decimation filter in the sigma delta converter can filter out the display line refresh noise. The decimation filter attenuates high frequency noise. Thus, the length and shaping (i.e., weights) of the decimation filter used for analog-to-digital conversion is selected to produce sufficient attenuation at the frequency of the display line refresh noise. When the noise is at a high frequency, such as with the display noise or other system interference noise, the decimation filter can filter this noise out within a subscan operation. The frequencies of the filter notches and overall attenuation profile of the overall filter (including subscan averaging filter, decimation filter, and analog filtering in the system) can be strategically designed based on the frequency or frequencies of the noise. 
     Those skilled in the art will recognize that other embodiments are not limited to a sigma delta ADC. A separate filter circuit can be used in combination with an ADC to filter out the display line refresh noise. Additionally or alternatively, the sense circuitry may not include the multiplexer  1002  in another embodiment. The force and reference signals can be input directly into the amplifier circuit  1010 . In such an embodiment, sensor matching or active temperature compensation may be needed. 
     Referring now to  FIG. 11 , there is shown a flowchart of a method of cancelling noise in force signals that are produced by a strain sensitive structure. Initially, as shown in block  1100 , the force signals are sampled using a sampling scheme that reduces or cancels out noise in the force signals. The timing of the sampling scheme can be designed to cancel out noise that is produced by one or more sources. As described earlier, the timing of the sampling scheme can cancel or reduce touch to display handoff noise, Johnson noise, background noise, system noise, and/or display line refresh noise. 
     After the force signals have been sampled according to the desired sampling scheme at block  1100 , the force signals can be processed further with one or more noise cancelling techniques to reduce or cancel noise (block  1102 ). As one example, a bucking technique can be used to cancel noise in the force signals. As another example, adaptive filtering may be used to reduce or eliminate noise in the force signals. Next, as shown in block  1104 , the amount of force that was applied to an input region is determined based on the force signals. 
     As described earlier, the sampling scheme disclosed herein can be used to reduce the amount of noise injected into force signals by one or more sources. The techniques may be used individually or in various combinations. In one example, the delay periods between subscan operations can be aligned with system interferences in combination with bias flipping to reduce both touch-to-display handoff noise and system noise. In another example, the delay periods and the time period t 1  between subscan pairs (the time period between every other subscan operation) can be selected to reduce system and background noise. Additionally or alternatively, in this example the time period t 2  may be used to reduce background noise produced at a different frequency. In yet another example, all of the techniques can be employed together to reduce noise from multiple sources. 
     Various embodiments have been described in detail with particular reference to certain features thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the disclosure. For example, a device other than a touch sensing device and/or a force sensing device can share at least a portion of the display area. By way of example only, a fingerprint sensor can use at least a portion of the top surface of the display as an input region. 
     Even though specific embodiments have been described herein, it should be noted that the application is not limited to these embodiments. In particular, any features described with respect to one embodiment may also be used in other embodiments, where compatible. Likewise, the features of the different embodiments may be exchanged, where compatible.

Metadata:
Filing Date: 20150519
Publication Date: 20190521
Grant Date: 20190521
Priority Date: 20150306
Inventors: AGARWAL, MANU
SMITH, JOHN STEPHEN
Assignee: APPLE INC
CPC Classifications: [{"code": "G06F2203/04106", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04106", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F2203/04106", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/04182", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/04166", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/04182", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/04166", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 56850528