Abstract:
An example method includes measuring a capacitance of an actuator and a conductive element when, responsive to a force applied to the actuator, the actuator is coupled to a reference voltage and deformed such that surface area of the actuator proximate to the conductive element increases. The example method includes determining the force applied to the actuator based on the measured capacitance.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to electronic circuits and in particular to circuits for sensing force. 
       BACKGROUND 
       [0002]    Force-sensing buttons have found recent widespread use in human interface devices such as gamepads for the entertainment consoles like the Sony PlayStation™ and Microsoft Xbox™. A conventional gamepad  100  is shown in  FIG. 1 . The conventional gamepad  100  comprises a housing  110 , having four force-sensing triggers  120 , a D-pad  130  with four force sensing buttons controlled by a left hand, four force sensing buttons  140  controlled by a right hand, and two thumbsticks  150  controlled by thumbs. The force sensing buttons comprise electronic force sensing actuators (in which the force applied to a button is sensed, rather than the binary state of a button) to provide variable force inputs to the console. In this conventional gamepad, there are twelve force sensing buttons/actuators. Typically each force sensing button/actuator output is translated in a six or eight bit value representing the force applied. 
         [0003]    One conventional implementation for a force sensing actuator is the use of a force sensing resistor, such as those sold by Interlink Electronics (cited in information disclosure statement). However the force sensing resistor solution is too expensive for many applications where cost is an important factor. Many purchasers of gamepads and other consumer products are very price sensitive, so having a low manufacturing cost is important. 
         [0004]    Another lower cost conventional implementation (which has been adopted by many gamepad manufacturers) is to use a resistive track printed on a printed circuit board (PCB). Printed circuit boards typically comprise a substrate, with one or more layers of copper traces on the surface or sandwiched between layers of substrate. To prevent corrosion and to prevent short circuits, the copper traces are coated with a thin film of “solder resist” except at the locations of pads or holes where components are to be soldered to the copper traces. In some cases, the copper traces may be gold plated. 
         [0005]    In some cases, PCBs also contain resistive carbon traces printed on one or both sides of the PCB. The resistivity of such traces may vary between a few ohms/square and several kilo ohms/square. Such carbon traces may be used for a variety of purposes, including preventing corrosion of exposed copper contacts and to implement a variable resistance in combination with an external actuator or wiper. 
         [0006]    The cost of a PCB is determined primarily by its area, the type of substrate material used, the number and size of holes in the PCB, and the number of layers of copper traces. The minimum width of the traces, and the minimum distance between traces also may significantly affect PCB cost, but the number of traces, or the percentage of the area of the PCB that is covered in copper are not significant factors affecting the cost of a PCB. 
         [0007]      FIG. 2  shows a conventional actuator button  200  such as one used in a gamepad or other control device. The button has a carbon-impregnated domed rubber actuator, which makes contact with a resistive carbon PCB track. As the button is pressed harder, the rubber dome deforms, progressively shorting out more of the printed carbon PCB track, reducing the end-to-end resistance of the track, as shown in  FIG. 2 . 
         [0008]    When the button is in the ‘rest’ position  210 , it is not in contact with a carbon track  250 , and resistive value of the track is shown as the resistor representation  260 . When the button is gently pressed it goes to position  220 , where the tip of the dome contacts the carbon track  250 , and shorts across a small portion of the track  250 . This is visible as the ‘shorted out’ portion of the resistor representation  265 . When the button is pressed more firmly as shown in position  230 , the tip of the dome deforms to become flatter and shorts out a larger portion of the track  250 . This is visible as the wider ‘shorted out’ portion of the resistor representation  270 . Finally, if the button is pressed hard as shown in position  240 , the tip of the dome deforms to become quite flat and shorts out a much wider portion of the track  250 , such that almost the entire track  250  is shorted out. This is visible as the widest ‘shorted out’ portion of the resistor representation  275 . 
         [0009]    The arrows in the drawing show the portion of the track which is not shorted out, and which is therefore resistive. The area between the arrows shows the area of the track which is shorted out. It can therefore be seen that as the rubber button is pressed harder, more of the track is shorted out, and the total resistance between the 2 ends of the track is reduced. The resistive track usually has a total resistance of a few kilo ohms, while the resistance of the conductive coating on the bottom of the rubber button is typically a few ohms at most. The resistance may be measured by placing a second resistor (for example 10K Ohms) in series with it to form a potentiometer, and measuring the output voltage from the potentiometer using an analog to digital converter (ADC). 
         [0010]    This conventional actuator button and resistive track of  FIG. 2  is somewhat less accurate than the force sensing resistor (FSR) approach, and has lower linearity. The main reason for the lower accuracy and non-linearity of the conventional actuator button and resistive track is the difficulty in printing a resistive track with a consistent resistivity along its length, and consistent resistivity from printed track to printed track. It is difficult to accurately control the thickness of the printed trace in a mass manufacturing process. However, absolute accuracy and linearity may not be important in many applications, and with calibration it is possible to give reasonably consistent results. Firmware may be used to calibrate for the non-linearity and also to calibrate for the changes in resistance as the rubber dome wears out with use. However, while the conventional actuator button and resistive track solution is less expensive than a force sensing resitor, it still costs several cents for each printed resistive trace on the PCB, and such costs can be significant in a consumer product with many force-sensing buttons (for example twelve buttons in the example of  FIG. 1 ). 
         [0011]      FIG. 3  shows a disassembled conventional gamepad  300 , with resistive carbon traces  310 , and conductive rubber dome actuators  320 . 
         [0012]      FIG. 4  shows a printed circuit board layout  400  of the conventional gamepad. The layout  400  shows resistive carbon printed traces  410 , PCB traces  420 , and solder resist (in this case blue, generally green in color)  430 . 
         [0013]    It would be desirable to have a less expensive force sensing button. A preferred force sensing button would be “free” (apart from the cost of the actuator itself) and provide linear sensing of force, with absolute accuracy that was consistent after calibration (low drift). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  illustrates a conventional gamepad device. 
           [0015]      FIG. 2  illustrates a conventional actuator button. 
           [0016]      FIG. 3  illustrates a disassembled conventional gamepad. 
           [0017]      FIG. 4  illustrates a printed circuit board layout of the conventional gamepad layout. 
           [0018]      FIG. 5  illustrates a side view of an improved force sensing actuator. 
           [0019]      FIG. 6  illustrates a plan view of an improved force sensing actuator 
           [0020]      FIG. 7  illustrates an alternative embodiment of the improved force sensing actuator. 
           [0021]      FIG. 8  illustrates another embodiment of the improved force sensing actuator. 
           [0022]      FIG. 9  illustrates another alternative embodiment of the improved force sensing actuator 
           [0023]      FIG. 10  illustrates another alternative embodiment of the improved force sensing actuator 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Described is a solution for a force sensing actuation that uses the electrical properties of a printed circuit board, together with a conductive-tip actuator as to make a force-sensing button at extremely low cost. 
         [0025]      FIG. 5  shows a side view  500  of the improved solution. The improved solution comprises a rubber actuator dome  510  which has a conductive layer (in one embodiment carbon) on the surface. In another embodiment, the entire actuator dome could be formed of conductive flexible material, or be impregnated with conductive material. The rubber actuator dome  510  is positioned above a PCB substrate  530 . A conductive layer  550  is formed on the PCB substrate  530 , and an insulating solder resist layer  520  is formed over the conductive layer  550 . In one embodiment, the conductive layer  550  is a PCB trace comprising copper or an alloy thereof. A trace  540  is formed on a lower layer or on the opposite side of the PCB from the conductive layer  550  and the solder resist  520 . The trace  540  is electrically isolated (i.e. not shorted to) from the conductive layer  550 , the trace  540  forms a contact  545  on the PCB on the same side as the solder resist  520 . The contact  545  is not fully covered by solder resist  520 , such that any conducting material pressed down onto the top surface of the substrate will make electrical contact with contact  545 . In another embodiment the contact  545  is exposed (i.e. there is no solder resist over it). 
         [0026]      FIG. 6  shows a plan view  600  of the arrangement of  FIG. 5 . Plan view  600  shows the PCB trace  550  (in one embodiment in a circular shape, but could have any shape). Located between the edges of the PCB trace is the contact  545 . In one embodiment this may be located approximately in the center of the PCB trace  550 . Trace  540  is shown as a dotted line, this trace will be electrically connected to the rubber actuator dome  510  (which is not shown in the plan view) when the dome is pressed into contact with the substrate. Trace  560  is the trace from the lower electrode which is coupled to conductive PCB trace  550 . 
         [0027]    The actuator  510  is formed of, impregnated with or coated with a conductive material with a low resistivity, for example carbon. The rubber actuator dome may be the same type as used in conventional solutions. Solder resist is commonly used to coat the copper traces of a PCB to protect it from short circuits and oxidation and is of relatively uniform thickness and reasonably constant relative permitivity, with a value of approximately 4 in one example. 
         [0028]    The value of the capacitance between two parallel plates is calculated as the permitivity of the material between the plates (the dielectric) multiplied by the overlapping area of the two plates, divided by the distance between the plates. Permitivity is commonly specified as two parts the permitivity of free space (epsilon-0 or E 0 ) and the relative permitivity of a particular material (gas, liquid, solid) known as epsilon-r or E r . Thus, the permitivity (epsilon) is E 0 *E r . 
         [0029]    A capacitor may be formed by the combination of a copper trace  550  (which acts as a lower plate), the solder resist  520  (which acts as a dielectric) and the conductive (e.g. carbon-printed) rubber actuator dome (which acts as an upper plate). As the actuator  510  is pressed down onto the PCB it will make contact with trace  540  through contact  545 ; as the actuator is pressed down with greater force, it will deform and a greater area of the conductive button will come into close proximity with the lower plate  550 , thus increasing the capacitance between plate  550  and trace  540 . A circuit on the board can be used to measure this capacitance. The output to be measured is a frequency that varies with capacitance. One example of such a circuit used to measure capacitance is a relaxation oscillator; this and other circuits for accurately measuring or detecting small changes in capacitance will be familiar to one skilled in the art. A processing element may read the output of this circuit and thus infer the force with which the button is being pressed. 
         [0030]    The shape of the conductive trace  550  or the solder resist  520  can be varied while preserving the function of the invention. In order to maximize the capacitance between the actuator  510  and the trace  550 , the trace  550  should generally cover the full area of contact of the actuator with the substrate when pressed with maximum force. In various configurations, the shape could be circle, square, rectangle, triangle, or any combination of these or other shapes. The shape could have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more sides, depending on how PCB layout software implements the conductive trace. PCB design/layout software may approximate a circular shape with a many sided shape, as true curves may be difficult to implement in PCB layout software. The conductive trace  550  may completely surround the contact  545 , or may partially surround (such as a horseshoe shape) the contact  545 . The conductive trace  550  may also be formed as a plurality of pieces (such as a pie chart shape) surrounding or partially surrounding the contact  545 . The contact  545  may be located somewhere inside the limits of trace  520 ; generally the contact  545  should be located at or close to the point on the substrate where the actuator first touches the PCB, i.e. where the actuator touches when pressed with least force. 
         [0031]    The improved solution operates in the following manner. In a first step when the actuator  510  is first touched by a user, it touches the sensor contact  545  which connects the actuator dome  510  to trace  540 . In one example, trace  540  may be connected to electrical ground, such that dome  510  becomes grounded when it touches contact  545 . This creates a small capacitance between the trace  550  and a ground voltage coupled to trace  540  and contact  545 . In a second step when the actuator is pressed more firmly it deforms and approaches a wider surface of the trace  550  causing the capacitance between trace  550  and electrical ground to increase. In a third step, a circuit measures the capacitance. In a fourth step a microcontroller samples the circuit output and determines the capacitance value. In a fifth step, a digital representation of that capacitance value is generated. In one embodiment, this digital representation may be a six bit or eight bit value. 
         [0032]      FIG. 7  shows an alternative embodiment  700  of the improved solution. In the embodiment  700 , a first trace  710  is formed in close proximity to a second trace  720 . Second trace  720  is coupled to ground. The traces  710  and  720  are electrically isolated, i.e. they are not shorted out. A layer of solder resist may be used to cover traces  710  and  720 . The actuator  510  in combination with the first trace  710  and second trace  720  and solder resist  520  form a three plate capacitor, with two plates  710  and  720  side by side and the actuator acting as the third plate. In this embodiment the actuator does not make DC contact with either plates, allowing easier mechanical alignment during manufacturing, but may reduce the possible capacitance between the plates. Trace  710  is coupled to the measurement device. 
         [0033]      FIG. 8  shows a further alternative embodiment  800  of the improved solution. In the embodiment  800 , a first trace  810 , a second trace  820  and a third trace  830  are formed. The first trace  810  is larger than either the second trace  820  or the third trace  830 . The third trace  830  is coupled to ground. The second trace  820  is coupled to a logic input and the first trace  810  is coupled to the measurement device. A layer of solder resist is formed over plate  810 , but plates  820  and  830  are not covered by solder resist. 
         [0034]    The embodiment  800  operates in the following manner. When the actuator makes contact with the plates  820  and  830 , the conductive actuator shorts them out and forms a DC connection to ground between the plates, which is detected by the logic input. Thus, the embodiment  800  forms both a combination switch and force sensing button. 
         [0035]    In another alternative embodiment  900  shown in  FIG. 9 , plate  910  is fully covered with solder resist  520 , and plate  920  is fully uncovered. When actuator  510  is pressed against the substrate, the actuator  510  is therefore grounded and, and a 2-plate capacitor is formed by  910  and  510  with solder resist acting as the dielectric. 
         [0036]    Another alternative embodiment  1000  is shown in  FIG. 10 . The embodiment  1000  comprises a first plate  1010 , a second plate  1020 , and a grounded trace  1030  placed between the first plate and the second plate. First plate  1010  and second plate  1020  are covered in solder resist, but trace  1030  is exposed (i.e. no solder resist). This embodiment  1000  is well suited for implementation on a single side PCB board. In another embodiment, a further trace  1040  is present and located between the first plate  1010  and second plate  1020 , where trace  1030  is grounded and trace  1040  is a logic output. 
         [0037]    Embodiments of the present invention are well suited to performing various other steps or variations of the steps recited herein, and in a sequence other than that depicted and/or described herein. In one embodiment, such a process is carried out by processors and other electrical and electronic components, e.g., executing computer readable and computer executable instructions comprising code contained in a computer usable medium. 
         [0038]    For purposes of clarity, many of the details of the improved force sensing actuator and the methods of designing and manufacturing the same that are widely known and are not relevant to the present invention have been omitted from the following description. 
         [0039]    It should be appreciated that reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Therefore, it is emphasized and should be appreciated that two or more references to “an embodiment” or “one embodiment” or “an alternative embodiment” in various portions of this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined as suitable in one or more embodiments of the invention. 
         [0040]    Similarly, it should be appreciated that in the foregoing description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.