Patent Publication Number: US-2021181885-A1

Title: Low-cost force sensor

Description:
TECHNICAL FIELD 
     The present embodiments relate generally to force sensing. 
     BACKGROUND OF RELATED ART 
     Input devices including force sensor devices (also commonly referred to as pressure sensor devices) are widely used in a variety of electronic systems. Force sensor devices may be used to provide interfaces for the electronic system. Some force sensor devices also have the ability to detect motion of the electronic system. For example, one or more force sensors positioned beneath the input surface may detect movement and/or vibration of the electronic system based, at least in part, on forces exerted on the input surface. Such forces may be interpreted as “tap” inputs to the electronic system. Accordingly, force sensor devices may be used as input devices for larger computing systems (such as touchpads integrated in, or peripheral to, notebook or desktop computers) and/or smaller computing systems (such as touch screens integrated in cellular phones). 
     SUMMARY 
     This Summary is provided to introduce in a simplified form a selection of concepts that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter. 
     A method and apparatus for force sensing is disclosed. One innovative aspect of the subject matter of this disclosure can be implemented in a sensor apparatus including a capacitor and detection circuitry operable in at least a first mode and a second mode. When operating the first mode, the detection circuitry is configured to measure a capacitance of the capacitor. When operating in the second mode, the detection circuitry is configured to monitor a piezoelectric response of the capacitor, where the piezoelectric response is determined based at least in part on the measured capacitance. 
     Another innovative aspect of the subject matter of this disclosure can be implemented in a method performed by an input device. The method includes steps of measuring a capacitance of a capacitor coupled to the input device; monitoring a piezoelectric response of the capacitor based at least in part on the measured capacitance; and detecting a force exerted on the input device based at least in part on the piezoelectric response of the capacitor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present embodiments are illustrated by way of example and are not intended to be limited by the figures of the accompanying drawings. 
         FIG. 1  shows an example input device within which the present embodiments may be implemented. 
         FIG. 2  shows block diagram of an input device, in accordance with some embodiments. 
         FIG. 3  shows an example force sensing apparatus, in accordance with some embodiments. 
         FIGS. 4A-4C  depict example voltage outputs of a force sensing apparatus in response to various stimuli. 
         FIG. 5  shows another force sensing apparatus, in accordance with some embodiments. 
         FIG. 6  shows another block diagram of an input device, in accordance with some embodiments. 
         FIG. 7  is an illustrative flowchart depicting an example force sensing operation, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth such as examples of specific components, circuits, and processes to provide a thorough understanding of the present disclosure. The term “coupled” as used herein means connected directly to or connected through one or more intervening components or circuits. The terms “electronic system” and “electronic device” may be used interchangeably to refer to any system capable of electronically processing information. Also, in the following description and for purposes of explanation, specific nomenclature is set forth to provide a thorough understanding of the aspects of the disclosure. However, it will be apparent to one skilled in the art that these specific details may not be required to practice the example embodiments. In other instances, well-known circuits and devices are shown in block diagram form to avoid obscuring the present disclosure. Some portions of the detailed descriptions which follow are presented in terms of procedures, logic blocks, processing and other symbolic representations of operations on data bits within a computer memory. 
     These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. In the present disclosure, a procedure, logic block, process, or the like, is conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, although not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer system. It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. 
     Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present application, discussions utilizing the terms such as “accessing,” “receiving,” “sending,” “using,” “selecting,” “determining,” “normalizing,” “multiplying,” “averaging,” “monitoring,” “comparing,” “applying,” “updating,” “measuring,” “deriving” or the like, refer to the actions and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     In the figures, a single block may be described as performing a function or functions; however, in actual practice, the function or functions performed by that block may be performed in a single component or across multiple components, and/or may be performed using hardware, using software, or using a combination of hardware and software. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described below generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. Also, the example input devices may include components other than those shown, including well-known components such as a processor, memory and the like. 
     The techniques described herein may be implemented in hardware, software, firmware, or any combination thereof, unless specifically described as being implemented in a specific manner. Any features described as modules or components may also be implemented together in an integrated logic device or separately as discrete but interoperable logic devices. If implemented in software, the techniques may be realized at least in part by a non-transitory processor-readable storage medium comprising instructions that, when executed, performs one or more of the methods described above. The non-transitory processor-readable data storage medium may form part of a computer program product, which may include packaging materials. 
     The non-transitory processor-readable storage medium may comprise random access memory (RAM) such as synchronous dynamic random access memory (SDRAM), read only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), FLASH memory, other known storage media, and the like. The techniques additionally, or alternatively, may be realized at least in part by a processor-readable communication medium that carries or communicates code in the form of instructions or data structures and that can be accessed, read, and/or executed by a computer or other processor. 
     The various illustrative logical blocks, modules, circuits and instructions described in connection with the embodiments disclosed herein may be executed by one or more processors. The term “processor,” as used herein may refer to any general-purpose processor, conventional processor, special-purpose processor, controller, microcontroller, and/or state machine capable of executing scripts or instructions of one or more software programs stored in memory. The term “voltage source,” as used herein may refer to a direct-current (DC) voltage source, an alternating-current (AC) voltage source, or any other means of creating an electrical potential (such as ground). 
       FIG. 1  shows an example input device  100  within which the present embodiments may be implemented. The input device  100  includes a processing system  110  and a sensor apparatus  120 . The input device  100  may be configured to provide input to an electronic system (not shown for simplicity). Examples of electronic systems may include personal computing devices (e.g., desktop computers, laptop computers, netbook computers, tablets, web browsers, e-book readers, and personal digital assistants (PDAs)), composite input devices (e.g., physical keyboards, joysticks, and key switches), data input devices (e.g., remote controls and mice), data output devices (e.g., display screens, printers, speakers, and earbuds), remote terminals, kiosks, video game machines (e.g., video game consoles, portable gaming devices, and the like), communication devices (e.g., cellular phones such as smart phones), and media devices (e.g., recorders, editors, and players such as televisions, set-top boxes, music players, digital photo frames, and digital cameras). 
     In some aspects, the input device  100  may be implemented as a physical part of the corresponding electronic system. Alternatively, the input device  100  may be physically separated from the electronic system. The input device  100  may be coupled to (and communicate with) components of the electronic system using various wired and/or wireless interconnection and communication technologies, such as buses and networks. Examples technologies may include Inter-Integrated Circuit (I 2 C), Serial Peripheral Interface (SPI), PS/2, Universal Serial bus (USB), Bluetooth®, Infrared Data Association (IrDA), and various radio frequency (RF) communication protocols defined by the IEEE 802.11 standard. 
     In the example of  FIG. 1 , the input device  100  may correspond to a force sensor device (e.g., also referred to as a “pressure sensor device”) configured to sense external forces and/or pressure exerted on the input device  100  and/or the electronic system. In some aspects, the external forces may be attributed to one or more input objects  140  in contact with (e.g., tapping or pressing) a surface of the input device  100  and/or the electronic system. Example input objects  140  include fingers, styli, and the like. Thus, the sensor apparatus  120  may be configured to generate force information representative of the force exerted by the input object  140  when making contact with the electronic system. In some other aspects, the external forces may be attributed to movement or vibration of the input device  100  and/or the electronic system (e.g., when the device is dropped onto a hard surface). Thus, the terms “force sensor device” and “motion sensor device” (e.g., also referred to as a “pressure sensor device”) may be used herein interchangeably. 
     The force information may be in the form of electrical signals representative of an amplitude (or change in amplitude) of the force applied to the input surface. In some embodiments, the sensor apparatus  120  may exhibit a piezoelectric effect under application of force, pressure, stress, vibration, or various forms of motion. For example, the sensor apparatus  120  may comprise a piezoelectric material which produces an electric charge in response to mechanical stress. The amount of charge produced by the piezoelectric material may be proportional to the amount of force or pressure applied. The accumulated charge can be measured as a voltage across the piezoelectric material. Thus, the sensor apparatus  120  may produce electrical signals in response to mechanical disturbances such as those produced by force, pressure, stress, vibration, and/or motion of the sensor apparatus  120 . 
     The processing system  110  may be configured to operate the hardware of the input device  100  to detect input from the sensor apparatus  120 . In some embodiments, the processing system  110  may operate the sensor apparatus  120  to detect forces exerted on the electronic system. For example, the processing system  110  may be configured to detect changes in the voltage output of the sensor apparatus  120 . In some aspects, one or more components of the processing system  110  may be co-located, for example, in close proximity to the sensing elements of the input device  100 . In some other aspects, one or more components of the processing system  110  may be physically separated from the sensing elements of the input device  100 . For example, the input device  100  may be a peripheral coupled to a computing device, and the processing system  100  may be implemented as software executed by a central processing unit (CPU) of the computing device. In another example, the input device  100  may be physically integrated in a mobile device, and the processing system  110  may correspond, at least in part, to a CPU of the mobile device. 
     In some embodiments, the processing system  110  may be implemented as a set of modules that are implemented in firmware, software, or a combination thereof. Example modules include hardware operation modules for operating hardware such as sensor electrodes and display screens; data processing modules for processing data such as sensor signals and positional information; and reporting modules for reporting information. In some embodiments, the processing system  110  may include sensor operation modules configured to operate sensing elements to detect inputs from the sensor apparatus  120  and/or mode changing modules for changing operation modes of the input device  100  and/or electronic system. 
     The processing system  110  may respond to inputs from the sensor apparatus  120  by triggering one or more actions. Example actions include changing an operation mode of the input device  110  and/or graphical user interface (GUI) actions such as cursor movement, selection, menu navigation, and the like. In some embodiments, the processing system  110  may provide information about the detected input to the electronic system (e.g., to a CPU of the electronic system). The electronic system may then process information received from the processing system  110  to carry out additional actions (e.g., changing a mode of the electronic system and/or GUI actions). 
     The processing system  110  may operate the sensor apparatus  120  to produce electrical signals indicative of forces (or lack of forces) exerted on the electrical system. The processing system  110  may perform any appropriate amount of processing on the electrical signals to translate or generate the information provided to the electronic system. For example, the processing system  110  may digitize analog signals received from the sensor apparatus  120  and/or perform filtering or conditioning on the received signals. In some aspects, the processing system  110  may subtract or otherwise account for a “baseline” associated with the sensor apparatus  120 . For example, the baseline may represent a state of the sensor apparatus  120  when no external force is detected. Accordingly, the information provided by the processing system  110  to the electronic system may reflect a difference between the voltage output detected from the sensor apparatus  120  and an associated baseline voltage. 
     In some embodiments, the processing system  110  may further determine positional information and/or force information for a detected input. The term “positional information,” as used herein, refers to any information describing or otherwise indicating a position or location of the detected input. Example positional information may include absolute position, relative position, velocity, acceleration, and/or other types of spatial information. Likewise, the term “force information,” as used herein, refers to any information describing or otherwise indicating a force exerted by an input object in contact with (e.g., tapping) a touch surface of the input device  100 . For example, the force information may be provided as a vector or scalar quantity (e.g., indicating a direction and/or amplitude). As another example, the force information may include a time history component and/or describe whether the force exerted by the input object exceeds a threshold amount. 
     In some embodiments, the input device  100  may include a touch screen interface (e.g., display screen) that at least partially overlaps the sensor apparatus  120 . For example, the sensor apparatus  120  may be disposed beneath the display screen, thereby providing a tap or touch interface for the associated electronic system. The display screen may be any type of dynamic display capable of displaying a visual interface to a user. Examples of suitable display screen technologies may include light emitting diode (LED), organic LED (OLED), cathode ray tube (CRT), liquid crystal display (LCD), plasma, electroluminescence (EL), or other display technology. In some aspects, the display screen may be controlled or operated, at least in part, by the processing system  110 . 
     As described above, the processing system  110  may detect a force or pressure exerted on the sensor apparatus  120  (e.g., an “input force”) based on a piezoelectric response. For example, the sensor apparatus  120  may include one or more piezoelectric transducers specifically designed and/or optimized to produce a consistent and measurable piezoelectric response under the application of force and/or pressure. However, piezoelectric transducers are manufactured to very precise specifications and therefore tend to be large and expensive. Aspects of the present disclosure recognize that some off-the-shelf circuit components and/or devices are formed from piezoelectric materials (e.g., ceramics, crystals, and the like), and may therefore be suitable for use as a low-cost force or motion sensor. 
     In some embodiments, the sensor apparatus  120  may comprise a capacitor having a dielectric formed from a piezoelectric material. For example, the capacitor may be a ceramic capacitor having one or more layers of ceramic material acting as the dielectric. Example suitable capacitors include, but are not limited to, multilayer ceramic capacitors (MLCCs), class 2 ceramic capacitors, and various other ferroelectric capacitors. The piezoelectric effect is considered a parasitic property of ceramic capacitors. Thus, in contrast to piezoelectric transducers, many ceramic capacitors are designed to reduce the piezoelectric effect. Aspects of the present disclosure recognize that an off-the-shelf capacitor may exhibit a relatively weak and/or inconsistent piezoelectric response to external stimuli (e.g., force, pressure, vibration, motion, etc.). Thus, in some embodiments, the input device  100  may include additional circuitry to amplify and/or measure the piezoelectric response of the sensor apparatus  120 . 
     Among other advantages, the embodiments described herein provide a low-cost force sensor by repurposing off-the-shelf circuit components for their piezoelectric properties. More specifically, aspects of the present disclosure may leverage a parasitic property of ceramic capacitors to detect input forces and/or motion of the input device  100  or electronic system. 
       FIG. 2  shows block diagram of an input device  200 , in accordance with some embodiments. The input device  200  may be one embodiment of the input device  100  of  FIG. 1 . Accordingly, the input device  200  may be configured to detect forces exerted on, or motion of, the input device  200  and/or an electronic system (not shown for simplicity) coupled to the input device  200 . The input device  200  includes a capacitor  210  and a processing system  220 . 
     The capacitor  210  may be formed from a piezoelectric material. For example, the capacitor  210  may be a ceramic capacitor having one or more layers of ceramic material acting as the dielectric. Example suitable capacitors include, but are not limited to, multilayer ceramic capacitors (MLCCs), class 2 ceramic capacitors, and various other ferroelectric capacitors. In some embodiments, the capacitor  210  may exhibit a piezoelectric effect under application of force, pressure, stress, vibration, or various forms of motion. For example, the capacitor  210  may produce an electric charge in response to mechanical stress. Aspects of the present disclosure recognize that capacitors with higher dielectric constants, lower voltage ratings, and higher layer counts tend to be more susceptible to the piezoelectric effect. 
     The processing system  220  may be configured to detect the charge accumulated on the capacitor  210  and process inputs based, at least in part, on the accumulated charge. For example, the charge may be detected as a voltage across the capacitor  210  (e.g., an output voltage). The processing system includes a force sensing module  222  and an input processing module  224 . The force sensing module  222  may convert the output voltage of the capacitor  210  to force information indicating an amount of force or pressure exerted on the capacitor  210 . For example, the change in output voltage may be proportional to the amount of force or pressure applied to the capacitor  210 . In some aspects, the force sensing module  222  may compare the output voltage to a baseline voltage of the capacitor  210  (e.g., when no external forces are exerted) to determine the occurrence and/or amplitude of force inputs. 
     In some aspects, the capacitor  210  may exhibit a change in output voltage in response to certain stimuli unrelated to force or motion. For example, an object (e.g., finger or stylus) in close proximity of the capacitor  210  may affect the output voltage without applying any force or pressure. This change in output voltage may be attributed to a capacitive response, rather than a piezoelectric response, of the capacitor  210 . In some embodiments, the force sensing module  222  may be further configured to distinguish a piezoelectric response from a capacitive response of the capacitor  210 . As described in greater detail below, the piezoelectric effect generally has a greater impact on the output voltage of the capacitor  210  than its capacitive response to external stimuli. Thus, in some aspects, the force sensing module  222  may distinguish the piezoelectric response of the capacitor  210  from its capacitive response based, at least in part, on the degree and/or rate of change in output voltage. 
     The input processing module  224  may process inputs based at least in part on the force information received via the capacitor  210 . For example, the input processing module  224  may convert the force information from the capacitor  210  to one or more inputs for the input device  200  and/or the electronic system. In some embodiments, the input processing module  224  may correlate the force information with an amount of force or pressure exerted by an input object on the input device  200 . For example, the force information may be generated in response to a user tapping or pressing on an input surface of the input device  200 . The input processing module  224  may interpret such force information as user inputs. In some aspects, user inputs may be associated with a desired action by the input device  200  and/or the electronic system. Example actions may include, but are not limited to, changing an operation mode of the input device  200  and/or GUI actions such as cursor movement, selection, menu navigation, and the like. 
     In some other embodiments, the input processing module  224  may correlate the force information with a movement or acceleration of the input device  200 . For example, the force information may be generated in response to the input device  200  being dropped, thrown, or pushed across a surface. The input processing module  224  may interpret such force information as motion inputs. In some aspects, one or more motion inputs may be associated with a desired action by the input device  200  and/or the electronic system. However, aspects of the present disclosure recognize that some motion inputs may be unintentional and/or undesired. Thus, in some other aspects, one or more motion inputs may be associated with a preemptive action, for example, to prevent damage to the input device  200  and/or electronic system. Example preemptive actions may include, but are not limited to, cutting off a power supply or otherwise disabling one or more components of the input device  200  and/or electronic system. 
     Although only one capacitor  210  is depicted in the example of  FIG. 2 , other implementations of the input device  200  may include two or more capacitors. In some embodiments, the input device  200  may include a matrix of addressable capacitors dynamically configured to transmit and/or receive sound waves or audio signals. In some aspects, one or more voltages may be applied to the matrix of capacitors to induce mechanical vibrations (e.g., using the inverse piezoelectric effect). The mechanical vibrations may be emitted as sound waves from the capacitors. In some other aspects, the capacitors may exhibit varying levels of piezoelectric effect in response to received sound waves. For example, the variations in piezoelectric response may be used for far-field voice tracking (e.g., to track a user of the input device  200 ). 
       FIG. 3  shows an example force sensing apparatus  300 , in accordance with some embodiments. The force sensing apparatus  300  may be one embodiment of the sensor apparatus  120  of  FIG. 1 . Accordingly, the force sensing apparatus  300  may be configured to generate force information  301  based on forces exerted on, or motion of, the force sensing apparatus  300  and/or an electronic system (not shown for simplicity) coupled to the force sensing apparatus  300 . The force sensing apparatus  300  includes a capacitor  310 , and a piezoelectric response (PR) monitoring module  320 . 
     The capacitor  310  may be formed from a piezoelectric material. For example, the capacitor  310  may be a ceramic capacitor having one or more layers of ceramic material acting as the dielectric. Example suitable capacitors include, but are not limited to, multilayer ceramic capacitors (MLCCs), class 2 ceramic capacitors, and various other ferroelectric capacitors. In some embodiments, the capacitor  310  may exhibit a piezoelectric effect under application of force, pressure, stress, vibration, or various forms of motion. For example, the capacitor  310  may produce an electric charge in response to mechanical stress. The voltage (V C ) across the capacitor can be represented as a function of the capacitance (C) and charge (Q) of the capacitor  310 : 
     
       
         
           
             
               V 
               C 
             
             = 
             
               Q 
               C 
             
           
         
       
     
     Since the charge Q is proportional to the force (F C ) or pressure exerted on the capacitor  310 , due to the piezoelectric effect, the above equation can be rewritten as: 
     
       
         
           
             
               V 
               C 
             
             ≈ 
             
               
                 F 
                 C 
               
               C 
             
           
         
       
     
     Thus, a capacitor  310  with a known capacitance C may be used to sense external forces F C  by measuring the voltage V C  of the capacitor  310 : 
         F   C   ≈C·V   C   (1)
 
     Aspects of the present disclosure recognize that many ceramic capacitors tend to produce a relatively weak piezoelectric response to external force or pressure. In some embodiments, the capacitor  310  may be disposed in a cavity or via of the force sensing apparatus  300  to increase air flow around the capacitor  310  and thereby improve its piezoelectric response. In some other embodiments, the force sensing apparatus  300  may include an amplifier  312  to amplify the voltage V C  of the capacitor  310 . In the example of  FIG. 3 , the amplifier  312  is depicted as an operational amplifier (op amp) having inverting (−) and non-inverting (+) terminals coupled to respective terminals of the capacitor  310 , where the difference in voltage between the inverting (V IN   − ) and non-inverting (V IN   + ) terminals is amplified at the output (V O ) of the amplifier  312  as a function of its gain (G): 
         V   O   =G ·( V   IN   +   −V   1N   − )= G·V   C   (2)
 
     In actual implementations, the amplifier  312  may comprise two or more op amps arranged such that the gain G is proportional to a resistance (R G ) (e.g., G=1+50KΩ/R G ). Resistors R 1  and R 2  drain a small amount of leakage current away from the amplifier  312  (which would otherwise accumulate on the capacitor  310 ). In some implementations, the resistors R 1  and R 2  may have substantially the same, if not identical, resistance values. 
     The PR monitoring module  320  is configured to monitor a piezoelectric response of the capacitor  310  and generate the force information  301  based, at least in part, on the piezoelectric response. As shown in Equation 1, the amount of force or pressure exerted on the capacitor  310  is proportional to the output voltage V O . In some embodiments, the PR monitoring module  320  may generate the force information  301  by subtracting a baseline voltage (associated with a quiescent state of the capacitor  310 ) from the output voltage V O . In some other embodiments, when generating the force information  301 , the PR monitoring module  320  may filter or otherwise distinguish a piezoelectric response of the capacitor  310  from a capacitive response. 
       FIGS. 4A-4C  show example voltage outputs of a force sensing apparatus in response to various stimuli. With reference for example to  FIG. 3 , the output voltages depicted in graphs  410 - 430  may be generated by the force sensing apparatus  300 . 
       FIG. 4A  depicts a time-varying voltage output of the force sensing apparatus  300  in response to an input object (e.g., an aluminum rod) being dropped onto an input surface (e.g., plastic housing) of the force sensing apparatus  300 . In the example of  FIG. 4A , the input object is dropped from a relatively close distance (e.g., ˜3 cm), at time t 1 , and bounces off the input surface several times before coming to a rest. Each bounce occurs at a respective one of the times t 1 -t 8 . As shown in  FIG. 4A , the initial impact of the input object (at time t 1 ) produces the greatest spike in output voltage V O , with successive bounces (at times t 2 , t 3 , t 4 , t 5 , t 6 , t 7 , and t 8 ) resulting in voltage spikes with diminishing amplitudes. The rate of change in the output voltage V O  tracks the damped harmonic motion of the input object bouncing on the input surface. Thus, the spikes in output voltage V O  at times t 1 , t 2 , t 3 , t 4 , t 5 , t 6 , t 7 , and t 8  can be attributed to the piezoelectric response of the capacitor  310 . 
       FIG. 4B  depicts a time-varying voltage output of the force sensing apparatus  300  in response to the force sensing apparatus  300  being dropped onto a hard surface (e.g., a table). In the example of  FIG. 4B , the force sensing apparatus  300  is dropped from a relatively close distance (e.g., ˜2.5 cm), at time t 1 , and bounces off the hard surface before coming to a rest. Each bounce occurs at a respective one of the times t 1 -t 8 . As shown in  FIG. 4B , the initial impact of the force sensing apparatus  300  (at time t 1 ) produces the greatest spike in output voltage V O , with successive bounces (at time t 2 , t 3 , and t 4 ) resulting in voltage spikes with diminishing amplitudes. The rate of change in the output voltage V O  tracks the damped harmonic motion of the force sensing apparatus  300  bouncing on the hard surface. Thus, the spikes in output voltage V O  at times t 1 , t 2 , t 3 , and to can be attributed to the piezoelectric response of the capacitor  310 . 
       FIG. 4C  depicts a time-varying voltage output of the force sensing apparatus  300  in response to an input object (e.g., a human finger) touching, tapping, or otherwise pressing against the input surface (e.g., plastic housing) of the force sensing apparatus  300 . In the example of  FIG. 4C , the input object makes contact with the input surface at three distinct times t 1 , t 2 , and t 3 . The first two touch events (at times t 1  and t 2 ) correspond to light finger touches, whereas the third touch event (at time t 3 ) correspond to a hard press. As shown in  FIG. 4C , the light touches (at times t 1  and t 2 ) produce relatively small spikes in output voltage V O , whereas the hard press (at time t 3 ) produces a much more significant spike in the output voltage V O . Since the light touches are unlikely to trigger a piezoelectric response by the capacitor  310 , the first two spikes in the output voltage V O  (at times t 1  and t 2 ) can be attributed to a capacitive response of the capacitor  310 . In contrast, the hard press is much more likely to trigger a piezoelectric effect. Thus, the final spike in output voltage V O  (at time t 3 ) can be attributed to the piezoelectric response of the capacitor  310 . 
     Aspects of the present disclosure recognize that the piezoelectric response of the capacitor  310  (e.g., at time t 3 ) may be distinguished or filtered from its capacitive response (e.g., at times t 1  and t 2 ) based on the amplitude of the output voltage V O  and/or the rate of change of the output voltage V O . In some embodiments, the PR monitoring module  320  may sense the piezoelectric response of the capacitor  310  when the output voltage V O  exceeds a threshold amount. In some other embodiments, the PR monitoring module  320  may sense the piezoelectric response of the capacitor  310  when the output voltage V O  exceeds a threshold rate of change. 
     As shown in Equation 1, the force information  301  depends not only on the voltage measured across the capacitor  310 , but also on the capacitance of the capacitor  310 . However, aspects of the present disclosure recognize that the capacitance values of ceramic capacitors tend to vary due to process and temperature. For example, two capacitors made to the same specification may have slightly different capacitance values. Even the same capacitor may exhibit different capacitance values at different times (e.g., under varying temperatures and/or operating conditions). As a result, changes in the measured output voltage V O  may be attributable to changes in the force exerted on the capacitor  310  and/or the capacitance of the capacitor  310 . To ensure consistent and accurate force information, it may be desirable to take into account any variations in the capacitance of the capacitor  310 . In some embodiments, a force sensing apparatus may include circuitry to measure the capacitance of the capacitor to be used in force sensing applications. 
       FIG. 5  shows another force sensing apparatus  500 , in accordance with some embodiments. The force sensing apparatus  500  may be one embodiment of the sensor apparatus  120  of  FIG. 1 . Accordingly, the force sensing apparatus  500  may be configured to generate force information  501  based on forces exerted on, or motion of, the force sensing apparatus  500  and/or an electronic system (not shown for simplicity) coupled to the force sensing apparatus  500 . The force sensing apparatus  500  includes a capacitor  510 , a PR monitoring module  520 , and a capacitance measuring module  530 . 
     The capacitor  510  may be formed from a piezoelectric material. For example, the capacitor  310  may be a ceramic capacitor having one or more layers of ceramic material acting as the dielectric. With reference for example to  FIG. 3 , the capacitor  510  may be an embodiment of the capacitor  310 . Thus, the capacitor  510  may exhibit a piezoelectric effect under application of force, pressure, stress, vibration, or various forms of motion. As described with respect to  FIG. 3 , the force F C  exerted on the capacitor  510  may be proportional to the capacitance C of the capacitor  510  and the voltage V C  across the capacitor  510 . 
     In some embodiments, the force sensing apparatus  500  may include an amplifier  512  to amplify the voltage V C  of the capacitor  510 . With reference for example to  FIG. 3 , the amplifier  512  may be one embodiment of the amplifier  312 . Thus, the output V O  of the amplifier may be a function of its gain G and the voltage V C  of the capacitor  510  (e.g., as shown in Equation 2). In some implementations, the gain G may be proportional to the resistance R G . Resistors R 1  and R 2  drain a small amount of leakage current away from the amplifier  512  (which would otherwise accumulate on the capacitor  510 ). In some implementations, the resistors R 1  and R 2  may have substantially the same, if not identical, resistance values. 
     The PR monitoring module  520  is configured to monitor a piezoelectric response of the capacitor  510  and generate the force information  501  based, at least in part, on the piezoelectric response. As described above, the amount of force or pressure exerted on the capacitor  510  is proportional to the output voltage V O . In some embodiments, the PR monitoring module  520  may generate the force information  501  by subtracting a baseline voltage (associated with a quiescent state of the capacitor  510 ) from the output voltage V O . In some other embodiments, when generating the force information  501 , the PR monitoring module  520  may filter or otherwise distinguish a piezoelectric response of the capacitor  510  from a capacitive response. 
     The capacitance measuring module  530  may be configured to measure the capacitance of the capacitor  510 . As described above, the capacitance of the capacitor  510  may change under varying temperatures and/or operating conditions. Moreover, as shown in Equation 1, the force information  501  may vary with respect to variations in the capacitance of the capacitor  510 . Thus, to ensure that the PR monitoring module  520  is able to generate consistent and accurate force information  501 , the capacitance measuring module  530  may provide the measured capacitance value  503  to the PR monitoring module  520 . 
     In some embodiments, the capacitance measuring module  530  may provide a current (I M ) to the capacitor  510  when measuring the capacitance value  503 . For example, the capacitor  510  may be switchably coupled to a current source comprising a reference voltage (V M ) having a known voltage potential and a resistor (R M ) having a known resistance value. In the example of  FIG. 5 , a tri-state buffer  532  is used as a switch between the current source (e.g., V M ) and the capacitor  510 . However, in other embodiments, any suitable switching means may be used (e.g., switches, transistors, logic gates, and the like). The tri-state buffer  532  may be activated by an enable signal  502  under the control of the capacitance measuring module  530 . 
     When measuring the capacitance of the capacitor  510 , the capacitance measuring module  530  may assert or activate the enable signal  502  to couple the capacitor  510  to the reference voltage V M . This causes the current I M  to flow through, and charge, the capacitor  510 . While the capacitor  510  is charging, the capacitance measuring module  530  may measure the voltage response of the capacitor  510  (e.g., via the output voltage V O ) to determine the capacitance value  503 . For example, the charging of the capacitor  510  will follow an RC time constant curve: 
     
       
         
           
             
               
                 V 
                 C 
               
                
               
                 ( 
                 t 
                 ) 
               
             
             = 
             
               
                 V 
                 M 
               
               ( 
               
                 1 
                 - 
                 
                   e 
                   
                     - 
                     
                       t 
                       
                         R 
                          
                         C 
                       
                     
                   
                 
               
               ) 
             
           
         
       
     
     In the equation above, C is the capacitance of the capacitor  510  and R is the series resistance. Since all voltage and resistor values in the sensor apparatus  500  are known with a high level of precision, the capacitance C can be determined from the voltage response curve. After determining the capacitance C of the capacitor  510 , the capacitance measuring module  530  may deassert the enable signal  502  (e.g., to decouple the current source from the capacitor  510 ) and provide the measured capacitance C to the PR monitoring module  520  (e.g., as the capacitance value  503 ). 
     Deasserting the enable signal  502  decouples the current source (e.g., V M ) from the capacitor  510  and allows the voltage of the capacitor  510  to discharge (e.g., to its baseline voltage). The discharging of the capacitor  510  may also follow an RC time constant curve. Thus, in some other embodiments, the capacitance measuring module  530  may determine the capacitance C of the capacitor based on the voltage response curve associated with the discharging of the capacitor  510 . 
     Aspects of the present disclosure recognize that the charging of the capacitor  510  (e.g., using the current I M ) affects the output voltage V O  of the force sensing apparatus  500 . As a result, it may be difficult (if not impossible) to accurately measure the piezoelectric response of the capacitor  510  while concurrently determining its capacitance. In some embodiments, the force sensing apparatus  500  may be configured to switch between a capacitance measuring mode and a piezoelectric response (PR) monitoring mode. In some aspects, the force sensing apparatus  500  may operate in the capacitance measuring mode at device startup and may subsequently operate in the PR monitoring mode thereafter. In some other aspects, the force sensing apparatus  500  may periodically switch between the capacitance measuring mode and the PR monitoring mode. 
     When operating in the capacitance measuring mode, the capacitance measuring module  530  may provide the current Inn to the capacitor  510  while monitoring the voltage response of the capacitor  510  to determine its capacitance value  503 . During this time, the PR monitoring module  520  may not monitor the piezoelectric response of the capacitor  510 . When operating in the PR monitoring mode, the PR monitoring module  520  may monitor the piezoelectric response of the capacitor  510  (e.g., to external forces) to determine the force information  501 . During this time, the capacitance measuring module  530  may not provide the current Inn to the capacitor  510 . In some embodiments, the PR monitoring module  520  may use the capacitance value  503  in calculating the force information  501 . 
       FIG. 6  shows another block diagram of an input device  600 , in accordance with some embodiments. The input device  600  may be one embodiment of the input device  110  of  FIG. 1  and/or the input device  200  of  FIG. 2 . Accordingly, the input device  600  may be configured to detect forces exerted on, or motion of, the input device  600  and/or an electronic system (not shown for simplicity) coupled to the input device  600 . The input device  600  includes a sensor interface  610 , a processor  620 , and a memory  630 . 
     The sensor interface  610  may be coupled to a sensor apparatus that is operable to detect forces or pressure exerted thereon. In some embodiments, the sensor apparatus may comprise a capacitor formed from a piezoelectric material. For example, the sensor apparatus may be a ceramic capacitor having one or more layers of ceramic material acting as the dielectric. The sensor interface  610  may be used to communicate with the capacitor. In some aspects, the sensor interface  610  may be configured to detect a voltage (or change in voltage) across the capacitor. In some other aspects, the sensor interface  610  may be configured to transmit signals to, and receive resulting signals from, the capacitor. 
     The memory  630  may include a non-transitory computer-readable medium (e.g., one or more nonvolatile memory elements, such as EPROM, EEPROM, Flash memory, a hard drive, etc.) that may store at least the following software (SW) modules:
         a mode selection SW module  631  to select an operating mode for the input device  600 , the operating modes include at least a capacitance sensing mode and a piezoelectric response (PR) monitoring mode;   a force sensing SW module  632  to measure a force exerted on the capacitor, the force sensing SW module  632  including:
           a capacitance measuring submodule  633  to measure a capacitance of the capacitor when operating in the capacitance sensing mode; and   a PR monitoring submodule  634  to monitor a piezoelectric response of the capacitor when operating in the PR monitoring mode; and   
           an input processing SW module  635  to process inputs for the input device  600  and/or the electronic system based, at least in part, on the piezoelectric response of the capacitor.
 
Each software module includes instructions that, when executed by the processor  620 , cause the input device  600  to perform the corresponding functions. The non-transitory computer-readable medium of memory  630  thus includes instructions for performing all or a portion of the operations described below with respect to  FIG. 7 .
       

     Processor  620  may be any suitable one or more processors capable of executing scripts or instructions of one or more software programs stored in the input device  600  (e.g., within memory  630 ). For example, the processor  620  may execute the mode selection SW module  631  to select an operating mode for the sensor apparatus. The processor  620  may also execute the force sensing SW module  632  to measure a force exerted on the capacitor. In executing the force sensing SW module  632 , the processor  620  may further execute the capacitance measuring submodule  633  and/or the PR monitoring submodule  634 . For example, the processor  620  may execute the capacitance measuring submodule  633  to measure a capacitance of the capacitor when operating in the capacitance sensing mode. The processor  620  may execute the PR monitoring submodule  634  to monitor a piezoelectric response of the capacitor when operating in the PR monitoring mode. Still further, the processor  620  may execute the input processing SW module  635  to process inputs for the input device  600  and/or the electronic system based, at least in part, on the piezoelectric response of the capacitor. 
       FIG. 7  is an illustrative flowchart depicting an example force sensing operation  700 , in accordance with some embodiments. With reference for example to  FIG. 6 , the operation  700  may be performed by the input device  600  to detect forces exerted on a capacitor. 
     The input device may measure a capacitance of a capacitor coupled to the input device ( 710 ). For example, the input device may provide a current to the capacitor when measuring the capacitance of the capacitor. With reference for example to  FIG. 5 , a current source (e.g., V M ) may be switchably coupled to the capacitor operating in a capacitance measuring mode. The provided current I M  charges the capacitor  510  to the reference voltage V M . While the capacitor  510  is charging, the input device may measure the voltage response of the capacitor to determine its capacitance value. For example, the charging of the capacitor will follow an RC time constant curve which can be used to determine the capacitance C of the capacitor. 
     The input device further monitors a piezoelectric response of the capacitor based at least in part on the measured capacitance ( 720 ). For example, the capacitor may be a ceramic capacitor having one or more layers of ceramic material acting as the dielectric. Example suitable capacitors include, but are not limited to, multilayer ceramic capacitors (MLCCs), class 2 ceramic capacitors, and various other ferroelectric capacitors. In some embodiments, the capacitor may exhibit a piezoelectric effect under application of force, pressure, stress, vibration, or various forms of motion. For example, the capacitor may exhibit a change in voltage in response to mechanical stress. The voltage depends on the capacitance of the capacitor. Thus, when monitoring the piezoelectric response, the input device may take into account the measured capacitance of the capacitor. 
     The input device may detect a force exerted on the input device based at least in part on the piezoelectric response of the capacitor ( 730 ). As described above, the amount of force or pressure exerted on the capacitor is proportional to the measured voltage across the capacitor. In some embodiments, the input device may generate force information by subtracting a baseline voltage (associated with a quiescent state of the capacitor) from the measured voltage of the capacitor. In some other embodiments, when generating the force information, the input device may filter or otherwise distinguish a piezoelectric response of the capacitor from a capacitive response. 
     Those of skill in the art will appreciate that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     Further, those of skill in the art will appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the aspects disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the disclosure. 
     The methods, sequences or algorithms described in connection with the aspects disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. 
     In the foregoing specification, embodiments have been described with reference to specific examples thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader scope of the disclosure as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.