Patent Publication Number: US-8978487-B2

Title: Capacitive force sensor with magnetic spring

Description:
BACKGROUND 
     Stylus pointing devices enable information to be input to a host electronic device. When the tip of the stylus is placed in close proximity to a drawing and/or display surface of the host device, the position of the tip may be determined by the host by a variety of methods, including the influence of the stylus on the electrical properties of the tablet (i.e., via electromagnetic induction, changes in electrical resistance, electrical capacitance, and the like); the optical properties of the tablet; or by ultrasonic positioning. 
     Some stylus pointing devices also provide a force, or pressure, output indicative of how hard a user is pressing the stylus against the drawing/display surface of the host electronic device. A variety of force sensors have been used. In many, the stress applied to the tip of the stylus is applied to an elastic element, resulting in a movement or strain of the element that, in turn, produces a change in an electrical property of a sensing circuit. In one approach, two plates of a capacitive sensor are held apart by a mechanical spring. The stress applied to the tip of the stylus compresses the spring and produces movement of one plate. However, this approach requires the use of a mechanical spring to bias the two plates of the capacitive sensor apart. However, mechanical springs are subject to fatigue, and spring rates may vary with temperature, age, etc. 
     Accordingly, it would be useful, therefore, to provide a capacitive sensor that does not require the use of a mechanical spring and is suitable for use in a stylus pointing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present disclosure will be described below with reference to the included drawings such that like reference numerals refer to like elements and in which: 
         FIG. 1  is a diagram of a force sensing stylus and a host electronic device, in accordance with exemplary embodiments of the present disclosure; 
         FIG. 2  is a diagrammatic representation of the tip end of a stylus, in accordance with an exemplary embodiment of the disclosure; 
         FIG. 3  is a further diagrammatic representation of the tip end of a stylus, in accordance with an exemplary embodiment of the disclosure; 
         FIG. 4  is a block diagram of system that includes a force sensing stylus and a host electronic device, in accordance with exemplary embodiments of the present disclosure; 
         FIG. 5  is a flow chart of a method for controlling an application executed on host electronic device, in accordance with certain embodiments of the present disclosure; 
         FIG. 6  is a flow chart of a method for producing a force signal characteristic of a force applied to a stylus, in accordance with certain embodiments of the present disclosure; 
         FIG. 7  is a diagrammatic representation of the tip end of a stylus, in accordance with a further embodiment of the disclosure; 
         FIG. 8  is a diagrammatic representation of the tip end of a stylus, in accordance with a still further embodiment of the disclosure; and 
         FIGS. 9-11  are block diagrams of exemplary capacitance sensing circuits, in accordance with embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     For simplicity and clarity of illustration, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. Numerous details are set forth to provide an understanding of the illustrative embodiments described herein. The embodiments may be practiced without these details. In other instances, well-known methods, procedures, and components have not been described in detail to avoid obscuring the disclosed embodiments. The description is not to be considered as limited to the scope of the embodiments shown and described here. 
     An exemplary aspect of the present disclosure relates to a force sensor operable to sense a force applied across a body. The force sensor includes a magnetic spring coupled between a body of the force sensor and a moveable element that resists a force applied across the body and the moveable element. The force sensor also includes a variable capacitor having first and second capacitor plates. The capacitance of the variable capacitor is dependent upon a position of the movable element with respect to the body. A sensing circuit, electrically coupled to the first and second capacitor plates of the variable capacitor, provides a force signal dependent upon a capacitance of the variable capacitor. 
     The moveable element may be one plate of the variable capacitor or may be a dielectric element that varies the permittivity of the space close to the plates of the variable capacitor. 
     For example, in one exemplary embodiment, the force sensor includes a variable capacitor that has a first capacitor plate that is moveably located with respect to the body of the force sensor and is configured to receive the force applied across the body. The force sensor also includes a second capacitor plate that is coupled to the body and located in proximity to the first capacitor plate. A sensing circuit, electrically coupled to the first and second capacitor plates of the variable capacitor, provides a force signal dependent upon a capacitance of the variable capacitor. The force applied across the body is resisted by a magnetic spring that produces a magnetic force between first and second magnets. 
     The force sensor may be used in a stylus pointing device, for example, where it produces a force signal characteristic of a force applied to the stylus. In this application, the force signal is produced by sensing a separation between a first magnet coupled to a body of the stylus and a second magnet coupled to a moveable tip of the stylus. The two magnets are configured to magnetically repel one another. 
     In one illustrative embodiment, the first magnet provides a first capacitor plate of the variable capacitor. In a further embodiment, the first magnet is non-conducting and is coupled to a conducting capacitor plate. The separation between magnets is sensed by sensing the capacitance of the variable capacitance. 
     A further aspect of the present disclosure relates to a stylus that is operable to provide a force signal to a host electronic device. The stylus includes a body with a moveable tip and a variable capacitance sensor. The variable capacitance sensor includes a first capacitor plate coupled to the moveable tip of the stylus, a second capacitor plate coupled to the body of the stylus and located in proximity to the first capacitor plate, and a sensing circuit electrically coupled to the first and second capacitor plates and operable to provide the force signal dependent upon a capacitance of the variable capacitor. A force applied to the moveable tip of the stylus is resisted by a magnetic force between the first and second magnets of a magnetic spring. 
     A further aspect of the present disclosure relates to a non-transitory computer-readable medium having computer-executable instructions that, when executed by a processor of a host electronic device, cause the host electronic device to control an application by receiving a stylus signal characteristic of a capacitance between a first capacitor plate coupled to a body of a stylus and a second capacitor plate coupled to a moveable tip of the stylus. The capacitance is dependent upon a separation of the first and second capacitor plates. In turn, the separation between the first and second magnetic plates is dependent upon a force applied to the moveable tip of the stylus and a magnetic force between first and second magnets of a magnetic spring. The force applied to the tip of the stylus is determined dependent upon the received stylus signal, and the application is controlled dependent upon the force applied to the tip of the stylus. The force applied to the tip of the stylus may be determined dependent upon calibration parameters stored in a memory of the host electronic device. 
     By way of example, an illustrative embodiment of a force sensing system  100  is shown in  FIG. 1 . In this example, a force sensor is included in a stylus pointing device  102 . The system includes the stylus  102  and a host electronic device  110 . In  FIG. 1 , the stylus  102  is operated by a user  104  and interacts with a drawing surface  108  of a host electronic device  110 . In the embodiment shown, the drawing surface  108  is combined with a visual display screen, although a separate display screen may be used. The host electronic device  110  may be a smart-phone, personal digital assistant (PDA), portable computer, tablet computer or any device utilizing a graphical user interface or drawing surface, for example. In operation, the tip  112  of the stylus  102  contacts the drawing surface  108 . The contact force is sensed by the stylus  102  and is communicated to the host electronic device  110  via a wired or wireless connection. 
       FIG. 2  is a diagrammatic representation of the tip end of a stylus, in accordance with an exemplary embodiment of the disclosure. A longitudinal section is shown. The moveable tip  112  of the stylus extends from the lower end of the stylus  102  and may be brought into contact with a drawing surface  108 . The tip  112  is coupled via coupling element  202  (shown as a shaft in the figure) to a first magnet  204  and a first capacitor plate  214 . The tip  112 , coupling element  202 , first magnet  204  and capacitor plate  214  are movable in a direction indicated by the arrow  206 . A second magnet  208  and a second capacitor plate  216  are coupled to the body of the stylus  102 . In one exemplary embodiment, the first and second magnets  204  and  208  are permanent magnets. The magnets may be disc-shaped, for example, although other magnet shapes and configurations may be used. The magnets may be electrically conducting or electrically insulating. In a further embodiment, the first and second magnetic elements are electrically conducting and are used as the capacitor plates. 
     The magnets are positioned such that a face of the first magnet  204 , having a first magnetic polarity, is adjacent a face of the second magnet  208  having the same magnetic polarity. In  FIG. 2 , the north poles (indicated by ‘N’ in the figure) of the permanent magnets are facing one another such the magnets repel one another. Optionally, a layer of dielectric material  210 , located between the first and second magnetic elements, may be used to maintain a minimum separation between the first and second magnetic elements. The dielectric layer  210  may be elastomeric or substantially rigid. 
     The maximum separation between the capacitor plates  214  and  216  is determined by the location of mechanical stop element  212 . 
     The magnetic force, indicated by the arrow  218 , between the first and second magnets,  204  and  208 , increases as the separation between the two plates decreases. The magnets therefore act a non-linear magnetic spring. The maximum separation, and hence the minimum magnetic spring force, is determined by the location of mechanical stop element  212 . 
     Once the force applied to tip  112  exceeds the minimum magnetic spring force, the first magnet  204  moves to a position where the magnetic spring force balances the contact force applied at the tip  112 . Thus, there is a direct relationship between the force applied to the tip and the resulting separation between the first and second capacitor plates. In  FIG. 2 , the force applied to the tip  112  is less than the minimum magnetic spring force, so the first capacitor plate  214  remains in contact with the stop  212 . 
       FIG. 3  is a further diagrammatic representation of the tip end of a stylus  102 , in accordance with an exemplary embodiment of the disclosure. In the configuration shown in  FIG. 3 , the force applied to the tip  112  is greater than the minimum magnetic spring force, so the first capacitor plate  214  is displaced from the stop  212  and the separation between the magnets (and the separation between the capacitor plates) is reduced. The applied force is balanced by the magnetic spring force when the first and second capacitor plates are separated by a distance d. 
     There is a direct relationship between the force applied to the tip  112  and the resulting separation, d, between the first and second capacitor plates  214  and  216 . In addition, the capacitance of the variable capacitor  302  formed by the first and second capacitor plates,  214  and  216 , is dependent upon the separation d between the plates. The variable capacitor  302  is electrically coupled to in a sensing circuit in which one or more properties of the sensing circuit, such as, for example, a voltage or a frequency or a phase response are dependent upon the capacitance. 
     For capacitor plates of area A, separated by a dielectric medium with permittivity ∈, the capacitance C is approximately related to the separation d by 
     
       
         
           
             
               
                 
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                         A 
                       
                       d 
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   1 
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     When the applied force F applied  is balanced by the magnetic spring force F magnetic , the forces are approximately related to the separation d by 
                       F   applied     =       F   magnetic     ≅       6   ⁢           ⁢     μ   0     ⁢     m   2         4   ⁢           ⁢   π   ⁢           ⁢     d   4             ,           (   2   )               
where μ 0  is the magnetic constant (permeability of free space) and m is the magnetic moment of the magnetic capacitor plates. Eliminating the separation d from equations (1) and (2) gives
 
     
       
         
           
             
               
                 
                   
                     F 
                     applied 
                   
                   = 
                   
                     
                       F 
                       magnetic 
                     
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                           6 
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                             μ 
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                             2 
                           
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                             C 
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                           4 
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                               ( 
                               
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     Equation (3) demonstrates an approximate relationship between the applied force F applied  and the capacitance C. Thus, a measurement of the capacitance C, or (equivalently) a measured quantity that is dependent upon the capacitance C, enables to the applied force F applied  to be determined. 
     In practice, the relationship between a measured quantity (dependent upon the capacitance C) and the applied force may be determined by calibration. For example, in equation (3) above, the quantity 
             p   =       6   ⁢           ⁢     μ   0     ⁢     m   2         4   ⁢           ⁢   π   ⁢           ⁢       (     ɛ   ⁢           ⁢   A     )     4               
may be considered to be a calibration parameter.
 
     A variety of capacitance sensing circuits are known to those of skill in the art. 
     In one illustrative embodiment, the first and second magnets function both as the plates of the capacitor and as a magnetic spring that biases the plates apart from one another. 
       FIG. 4  is a block diagram of system  100  that includes a stylus  102  and a host electronic device  110 , in accordance with exemplary embodiments of the present disclosure. The host electronic device  110  includes a processor  402  coupled to a memory  404 . The processor  402  is also coupled to a display driver  406  that is used to render images on a screen  408 . The screen  408  may be integrated with the drawing surface  108 . The drawing surface  108  may be used to sense a location of the stylus  102 . The memory  404  may be used to store an operating system and various user applications that may be executed on the processor  402 . The operating system and user applications control the processor to display elements of a graphical user interface on the screen  408 . The stylus  102  may be used to interact with the displayed graphical user interface to provide input to the operating system or other user applications executed on the processor  402 . 
     The stylus  102  includes a moveable tip  112 , motion of which alters the capacitance of a variable capacitor  302 . A sensing circuit  410 , which is electrically coupled to the variable capacitor  302 , outputs a stylus force signal to a transmitter  412 . The sensing circuit  410  may be, for example, an oscillator circuit having a frequency dependent upon the capacitance of the variable capacitor. A frequency signal produced by the oscillator may be supplied to a frequency-to-voltage converter, a counter that counts the period of the frequency signal. The resulting voltage or count may be used to generate a digital signal for output. Alternatively, the frequency signal may be used to modulate a radio frequency (RF) signal for wireless transmission via electromagnetic, ultrasonic, optical or other communication path. The stylus force signal  416  is transmitted to a communication circuit  414  of the host electronic device  110 . The stylus force signal  416  is characteristic of the force applied to the tip  112  of the stylus  102  and may be used as an input to control software applications executed on the processor  402  of the host electronic device  110 . 
       FIG. 5  is a flow chart of a method  500  for controlling an application on host electronic device, in accordance with certain embodiments of the present disclosure. The method may be implemented on the host electronic device by executing instructions stored on a non-transitory computer-readable medium, for example. Following start block  502  in  FIG. 5 , execution of an application is started on the host electronic device at block  504 . A stylus signal, characteristic of a capacitance between first and second capacitor plates of the stylus, is received by the host electronic device at block  506 . The capacitance is dependent upon a force applied to the moveable tip of the stylus and a magnetic force between two magnets of a magnet spring. At block  508  the force applied to the tip of the stylus is determined dependent upon the received stylus signal. The force may also depend upon calibration parameters stored in a memory of the host electronic device. At block  510 , the application is controlled dependent upon the force applied to the tip of the stylus. For example, in a computer drawing application, one or more properties of a line being drawn, such as the width, saturation, texture, style etc., may be varied dependent upon the force applied to the tip of the stylus. In another example, the force applied to a control of a graphical user interface may be used to adjust an application parameter dependent upon the force signal. The time history of the force may also be used. For example, tapping the stylus produces a characteristic time waveform that may be recognized and used to control aspects of the application. If execution of the application is terminated, as depicted by the positive branch from decision block  512 , the method terminates at block  514 . Otherwise, as depicted by the negative branch from decision block  512 , flow returns to block  506 . 
       FIG. 6  is a flow chart of a method  600  for producing a force signal characteristic of a force applied to a stylus, in accordance with certain embodiments of the present disclosure. Following start block  602 , a force applied to a moveable tip of the stylus is transferred to a first magnet of a magnetic spring at block  604 . At block  606 , the deflection of the magnetic spring is sensed. The deflection may be sensed, for example, by sensing a capacitance that is varied by a moveable element that is coupled to the moveable tip of the stylus and moves with the first magnet. The first and second magnets are biased apart by their respective magnetic fields, such that the separation between the magnets is dependent upon the force applied to the moveable tip of the stylus. In one exemplary embodiment, the first and second magnets form first and second plates of a variable capacitor and the separation between the first and second magnets is sensed by sensing a capacitance of the variable capacitor. A stylus signal, characteristic of the force applied to the moveable tip, is produced at block  608  and, at block  610 , the stylus signal is sent to a host electronic device, where it may be used, for example, to control a software application executing on the host electronic device. The method terminates at block  612 . 
       FIG. 7  is a diagrammatic representation of the tip end of a stylus, in accordance with a further embodiment of the disclosure. The tip  112  of the stylus is coupled, via coupling element  202 , to a first magnet  204  and a movable element  702 . The movable element  702  may include a high-k dielectric material or metal, for example. The surface of the dielectric element  702  that contacts the capacitor plates  214  and  216  is electrically insulating. 
     In operation, the elements  112 ,  202 ,  204  and  702  move in unison. A second magnet  208  is coupled to the body of the stylus  102 . In this embodiment, the coupling element  202  passes through a hole in the second magnet  208  and the second magnet supports sliding of the coupling element  202 . The dielectric element  702  is located in proximity to first and second capacitor plates,  214  and  216 , which are attached to the body of the stylus  102 . In operation, since the moveable plate has a different permittivity to air, movement of the tip  112  moves the dielectric element  702  and alters the effective permittivity ∈ in proximity to the capacitor plates. When a metal moveable element  702  is used, the capacitance C is approximately related to the separation d between the moveable element and the capacitor plates by 
     
       
         
           
             
               
                 
                   C 
                   = 
                   
                     
                       
                         ɛ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         A 
                       
                       
                         2 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         d 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In this embodiment, neither capacitor plate moves with the tip, which results in a more robust force sensor. 
     The capacitor plates  214  and  216  are electrically coupled to a sensing circuit  410 . The sensing circuit  410  outputs a stylus force signal to a transmitter  412 . The force signal may be transmitted to a communication circuit of a host electronic device. The force signal is characteristic of the force applied to the tip  112  of the stylus  102  and may be used as an input to control software applications executed on the host electronic device. 
       FIG. 8  is a diagram of a further embodiment, in which at least a portion of each fixed capacitor plate,  214  and  216 , is orientated along the length of the stylus and the dielectric element  702  moves between the capacitor plates. 
     When the plates have length l, and are separated by a distance d, the capacitance is approximated by 
                     C   =             ɛ   0     ⁡     (     l   -   x     )       d     +       ɛ   ⁢           ⁢   x     d       =         x   ⁡     (     ɛ   -     ɛ   0       )       d     +         ɛ   0     ⁢   l     d           ,           (   5   )               
where x is length of capacitor plate occupied by the moveable element, ∈ is the permittivity of the moveable element and ∈ 0  is the permittivity of air. Thus, apart from an offset, the capacitance varies linearly with the position of the moveable element,
 
     Other configurations will be apparent to those of skill in the art. For example, fixed concentric capacitor plates may be used with a dielectric ring moving between them. 
       FIG. 9  is a block diagram of an example of a sensing circuit in accordance with an exemplary embodiment of the invention. The sensing circuit  410  includes an oscillator circuit  902  that is electrically coupled to the variable oscillator  302 . The frequency signal  904  output from the oscillator circuit depends upon the capacitance of the variable capacitor  302 . The frequency signal  904  is converted to a voltage signal  906  in frequency-to-voltage converter  908 . The voltage signal  906  is converted to a digital signal in analog-to-digital converter (ADC)  910 . The resulting digital signal  912  is supplied to a force module  914  that converts the frequency dependent digital voltage signal  912  into a force signal. The resulting force signal  916  is output. Alternatively, the frequency dependent digital voltage signal  912  may be output to a host electronic device that performs the frequency-to-force conversion. 
       FIG. 10  is a block diagram of a further example of a sensing circuit in accordance with an illustrative embodiment of the invention. The sensing circuit  410  includes an oscillator circuit  902  that is electrically coupled to the variable oscillator  302 . The frequency signal  904  output from the oscillator circuit depends upon the capacitance of the variable capacitor  302 . The period of the frequency signal  904  is counted by counter  1002 , which may, for example, count a number of clock cycles between zero crossings in the frequency signal  904 . The resulting counter value  1004  is supplied to a force module  1006  that converts the count value into a force signal  1008  for transmission to a host electronic device. Alternatively, the counter value  1004  may be output to a host device that performs the count-to-force conversion. 
       FIG. 11  is a block diagram of a further example of a sensing circuit in accordance with an exemplary embodiment of the invention. The sensing circuit  410  includes an oscillator circuit  902  that is electrically coupled to the variable oscillator  302 . The frequency signal  904  output from the oscillator circuit depends upon the capacitance of the variable capacitor  302 . A radio frequency (RF) oscillator  1102  generates an RF signal that is modulated by frequency signal  904  in signal multiplier  1104 . The resulting modulated RF signal  1106  is provided as output. This signal may be communicated to a host device, where it may be demodulated to recover the frequency signal and, hence, the applied force. 
     Other sensing circuits will be apparent to those of ordinary skill in the art. 
     It will be appreciated that any module or component disclosed herein that executes instructions may include or otherwise have access to non-transient and tangible computer readable media such as storage media, computer storage media, or data storage devices (removable or non-removable) such as, for example, magnetic disks, optical disks, or tape data storage. Computer storage media may include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information, such as computer readable instructions, data structures, program modules, or other data. Examples of computer storage media include RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by an application, module, or both. Any such computer storage media may be part of the server, any component of or related to the network, backend, etc., or accessible or connectable thereto. Any application or module herein described may be implemented using computer readable/executable instructions that may be stored or otherwise held by such computer readable media. 
     The implementations of the present disclosure described above are intended to be merely exemplary. It will be appreciated by those of skill in the art that alterations, modifications and variations to the illustrative embodiments disclosed herein may be made without departing from the scope of the present disclosure. Moreover, selected features from one or more of the above-described embodiments may be combined to create alternative embodiments not explicitly shown and described herein. 
     The present disclosure may be embodied in other specific forms without departing from its spirit or essential characteristics. The described exemplary embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the disclosure is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.