Abstract:
An electroacoustic transducer includes a housing and a moving portion adapted to move relative to the housing. The moving portion has a first surface and includes a first electrode. A second electrode on a first surface of the housing is located proximate to a first region of the first electrode, and a third electrode on the first surface of the housing is located proximate to a second region of the first electrode. A capacitance between the first and second electrodes and a capacitance between the first and third electrodes vary similarly with displacement of the moving portion relative to the housing. An impedance buffer is coupled to the second electrode and the third electrode.

Description:
BACKGROUND 
       [0001]    This disclosure relates to an electromechanical transducer with an integrated sensor for measuring displacement and its derivatives. 
         [0002]    An electromechanical transducer with such an integrated sensor permits feedback control systems to directly measure displacement of the moving portion of an electromechanical transducer. Displacement measurements can be derived to obtain velocity, acceleration, and jerk. One or more of these measurements can be directly or indirectly used by a feedback control system for system control. 
       SUMMARY 
       [0003]    In general, in some aspects, an electroacoustic transducer includes a housing and a moving portion located proximate to the housing and adapted to move relative to the housing. The transducer also includes a displacement sensor, which includes a first electrode adhered to the moving portion of the transducer, a second electrode on a first surface of the housing, located proximate to a first region of the first electrode, and a third electrode on the first surface of the housing, located proximate to a second region of the first electrode. A first capacitance between the first electrode and the second electrode and a second capacitance between the first electrode and the third electrode each vary similarly with a displacement of the moving portion relative to the housing. An impedance buffer is coupled to the second electrode and the third electrode. 
         [0004]    Implementations may include one or more of the following. The change in distance between a first surface of the housing and the first surface of the moving portion resulting from movement of the moving portion is substantially uniform over the area of the second and third electrodes. The impedance buffer may include a bias voltage source providing a fixed charge to at least one of the electrodes of the displacement sensor, and an amplifier amplifying a change in voltage between the first and second electrodes to produce an output voltage between first and second signal outputs. The moving portion may be metal and may include the first electrode. The moving portion may include a diaphragm. 
         [0005]    The first surface of the housing may be a surface of a printed circuit board, and the second and third electrodes may be formed from metal areas on the printed circuit board. The housing may include a basket. The housing may include a pole piece. The moving portion may include a voice coil mechanically coupled to a diaphragm. The moving portion may include a magnet mechanically coupled to a diaphragm. The layer of metal of the first electrode may include a coating on a non-conductive substrate. 
         [0006]    The transducer may include a compression-type electroacoustic transducer, with the housing including a phase plug and the second electrode and third electrode formed from metal on a surface of the phase plug. The phase plug may include a plurality of distinct parts, and the second and third electrode may be formed from layers of metal conforming to portions of a surface of one of the plurality of parts of the phase plug, or they may be formed from solid metal portions of one of the plurality of parts of the phase plug. The first surface of the housing may be a surface of a block of conductive material, and the second and third electrodes may be formed from portions of the block that are electrically insulated from each other. The first surface of the housing may be a surface of a block of non-conductive material, and the second and third electrodes may be formed from layers of metal adhered to the block of non-conductive material. 
         [0007]    The amplifier may include a transistor having its gate coupled to a first terminal of the displacement sensor and its source and drain coupled to a first signal output and a second signal output. The bias voltage source may include an external power source having a first terminal coupled to the drain of the transistor and a second terminal coupled to a second terminal of the displacement sensor, the bias voltage being applied to the at least one of the electrodes via gate leakage of the transistor. The bias voltage source may include a permanently charged material within the at least one of the electrodes. 
         [0008]    In general, in another aspect, an electroacoustic transducer includes a housing and a diaphragm located proximate to the housing and adapted to move relative to the housing. The transducer also includes a displacement sensor, which includes a first electrode adhered to the housing, a second electrode on a first surface of the diaphragm, located proximate to a first region of the first electrode, and a third electrode on the first surface of the diaphragm, located proximate to a second region of the first electrode. A first capacitance between the first electrode and the second electrode and a second capacitance between the first electrode and the third electrode each vary similarly with a displacement of the moving portion relative to the housing. An impedance buffer is coupled to the second electrode and the third electrode. 
         [0009]    The second and third electrode may each include a layer of metal adhered to a top surface of the diaphragm facing away from the first surface of the housing, the diaphragm may be attached to the housing by a ring surrounding a periphery of the diaphragm, and electrical connections to the second and third electrodes may be made via the ring. The second and third electrode may each include a layer of metal adhered to a bottom surface of the diaphragm facing towards the first surface of the housing, and electrical connections to the second and third electrodes may be made via the housing where the housing contacts an outer periphery of the diaphragm. The second and third electrodes may each include a layer of metal adhered to a bottom surface of the diaphragm facing towards the first surface of the housing, and electrical connections to the second and third electrodes may be made where a voice coil is mechanically coupled to the diaphragm. 
         [0010]    Advantages include sensing the displacement of the moving structure without contacting it, so that the mechanical dynamic performance of the transducer is not substantially changed by the measurement. An integrated sensor may work over a broader frequency range and with lower noise than a discrete sensor. 
         [0011]    Other features and advantages will be apparent from the description and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1A  shows a cross-sectional plan view of an electroacoustic transducer. 
           [0013]      FIGS. 1B and 9  show exploded cross-sectional plan views of electroacoustic transducers 
           [0014]      FIGS. 2 and 8  show cross-sectional isometric views of electroacoustic transducers. 
           [0015]      FIGS. 3 ,  6 A, and  6 B show close-up isometric views of an integrated sensor in an electroacoustic transducer. 
           [0016]      FIGS. 4A through 4D  show close-up cross-sectional plan views of an electrical connection between components. 
           [0017]      FIGS. 5 ,  7 , and  10  show schematic circuit diagrams. 
       
    
    
     DESCRIPTION 
       [0018]    An electromechanical transducer includes an integrated displacement sensor for directly measuring displacement (and its derivatives) of the transducer. Such a transducer can be advantageous in a feedback control system where perturbations to the transducer are corrected by the control loop. For example, an electroacoustic transducer ( 10 ) with an integrated sensor is shown in  FIGS. 1A ,  1 B, and  2 . Transducer  10  includes a diaphragm  12 , a voice coil  14 , a magnetic assembly  16 , a basket  18 , and an integrated capacitive sensor  100 . 
         [0019]    When electrical current is applied to the voice coil  14 , it interacts with the magnetic field of the magnetic assembly  16  to produce the forces that move the voice coil  14  and diaphragm  12  relative to the magnetic assembly  16  and basket  18  to produce acoustic radiation. In some examples, the voice coil  14  and at least part of the magnetic assembly  16  are reversed, such that the magnetic assembly moves the diaphragm and the voice coil remains stationary relative to the basket. In the particular type of transducer shown, the diaphragm includes a dome  20  and a surround or suspension  22 . In other types of transducers, a cone may be used to provide additional radiating surface area. In this example, the integrated capacitive sensor  100  directly measures displacement of the diaphragm. 
         [0020]    Referring again to  FIGS. 1A-1B  and  2 , the magnetic assembly  16  includes a ring magnet  24 , a cup  26 , and a top plate or coin  28 . Other motor structure geometries may be used, depending on the particular application of the transducer. A hole  30  through the magnetic assembly allows air compressed on the back side of the diaphragm to escape out the back of the transducer. In some transducers, a limiter  32  seated atop the top plate physically limits the downward range of motion of the diaphragm, and has holes  34  to allow air to flow between the back side of the diaphragm and the hole  30  though the magnetic assembly. The limiter may be separated from the top plate  28  by a washer  36 . A ring  38  anchors the outer periphery of the diaphragm  12  to the basket  18 . The particular physical structures shown here are for illustration only, as the invention described below may be applicable to any type of electroacoustic transducer, however constructed. The physical components that support the active parts of the transducer but do not themselves contribute to the acoustic function aside from being present in the environment, such as the basket  18 , washer  36 , and limiter  32 , are referred to generically as the “housing.” 
         [0021]    The capacitive sensor  100  measures displacement of the diaphragm. This sensor is shown enlarged in  FIG. 3 . The capacitive sensor  100  includes a layer of metal  102  on the limiter, another layer of metal  104  on the diaphragm, circuitry  106  and circuit board  108 , and various electrical connections between these components. In transducers lacking the limiter  32 , a plate may be added or another structure within the transducer may be used to support the metal layer  102 . In the example of  FIG. 3 , the circuitry  106  and circuit board  108  are shown inside the limiter, with a wire  110  providing a connection to outside circuitry through the hole  30  through the magnetic assembly. In other examples, the circuitry and any circuit board may be located farther from the metal layers, or outside the transducer entirely. Electrical connections  112  between the metal layer  102  and the circuit board may be made through a hole  113  through the limiter  32 . Only one electrical component on the circuit board  108  is shown, but additional components may be provided on the circuit board in other implementations. An example circuit that may be implemented on the circuit board is described below in connection with  FIG. 5 . 
         [0022]    The layer of metal on the limiter (e.g., layer  102 ) is referred as the back plate, while the layer of metal on the diaphragm (e.g., layer  104 ) is referred as the front plate (of a two plate capacitor). In operation, a bias voltage is applied across the plates, and the circuit elements react to the change in capacitance between the plates, which varies with the displacement of the diaphragm, to produce a voltage that is proportional to the change in the displacement. Specifically, the charge on the plates is held constant by the bias voltage, so the changing capacitance changes the voltage across the plates. The change in voltage across the plates is then measured as an indication of displacement. The circuit serves as an impedance buffer to convert the signals produced by the sensor to signals usable by external circuitry. 
         [0023]    The circuitry may be coupled to the back plate in various ways, depending on the construction of the back plate and the limiter. In one example, as shown in  FIG. 4A , the entire limiter  32  is made of metal, such as copper. In this example, no separate component is needed to serve as the back plate, and the connection between the limiter  32  and the circuit board  108  may be made directly, without requiring any wires. For example, such connections could be made by locating contact pads of the circuit board  108  at the points where the limiter  32  meets the circuit board, and solder or conductive paste used to attach the two. In another example, as shown in  FIG. 4B , the back plate  102  is formed by a layer of metal on top of the limiter  32 , and a conductive path through the limiter, such as a channel  202  filled with solder or other conductive material electrically coupled to the back plate  102 , provides a connection point that can be connected to the circuit board  108 . In the example of  FIG. 4B , the connection point is spaced from the circuit board  108  and connected to it by a lead  204 . In other examples, the conductive path extends to a part of the limiter already in contact with the circuit board, so no lead is needed, as in the metallic limiter example of  FIG. 4A . In other examples, a conductive pin may extend from the circuit board into the channel  202 . In yet another example, as shown in  FIG. 4C , the back plate  102  is again provided by a layer of metal on top of the limiter  32 , and the connection between the back plate and the circuit board  108  is made by a lead wire  206  extending through an opening  208  in the limiter. The back plate  102  in the examples of  FIGS. 4B and 4C  may be formed in any conventional manner of placing a metal layer on another object, including application of foil, metallic paint, electroplating, or vapor deposition, to name a few examples. 
         [0024]    In another example, as shown in  FIG. 4D , the limiter itself is formed from a printed circuit board  210 , with the back plate provided by a metal layer  212 , and the circuitry  106  and any other electronics mounted to the opposite side of another layer  214  of the combined limiter/circuit board  210 . The metal layer  212  is shown having a curvature, and there are advantages to this, as explained below. In some examples, it may be preferred to keep the circuit board  210  fiat and compensate for the benefits provided by the curvature in other ways. A conductive pathway  216 , e.g., a standard via, connects the back plate layer  212  to the other layer  214  in a conventional manner for making connections between circuit board layers. Although the connection between the plate  102  and the circuit board  108  is shown in the center in each example, it may be located at other positions, or multiple connections may be made. In the example shown, a void beneath the circuit board  210  accommodates circuit elements such as the circuitry  106 . If such a void is not present in a given implementation, stand-offs may be used to create a space for such components between the circuit board and the structure beneath. Each of these different options may be advantageous in different situations, dependent on such factors as the sizes or curvatures of the components involved and the manufacturing processes used. For example, more discrete components may be used in a larger transducer, where there is more room, and more integrated components may be used in a smaller transducer, though the reverse may be advantageous in some situations. 
         [0025]    In some examples, the entire front surface of the diaphragm, that is, the surface facing the outside of the transducer, is metallized, though only the portion aligned with the metal portion of the back plate contributes significantly to the measured capacitance. Metallized areas surrounding the aligned portions may contribute some small effects. The electrical connection to that active portion is made through the remaining metallized region that extends to the outer edge of the diaphragm. The ring  38  (shown in  FIG. 2 ) used to attach the peripheral edge of the diaphragm to the basket is made of copper or another suitable conductive material, such that when the entire diaphragm is metallized, the electrical connection may be made at the ring  38 . This avoids the need to connect a wire lead to a moving part, a source of both distortion and failures in many loudspeakers. In some examples, the metallization may be masked to provide a connection to the outer rim without covering the entire surface. The metallized layer of the diaphragm may also be connected by a passage through the diaphragm at the point where it is joined to the voice coil, or formed on the bottom side of the diaphragm in the first place, allowing the electrical connection to the metallized layer to be made near the voice coil, the same point that other connections are made to the moving parts of the driver. It has been found that a metal layer on the diaphragm as thin as about 150 Å provides sufficient capacitance to reliably measure the displacement of the diaphragm while adding a negligible amount of moving mass. In some examples, the diaphragm material is sufficiently thin that the choice of side for metallization does not appreciably affect the capacitance or the responsiveness of the sensor, and which side of the diaphragm is metallized may be based on manufacturing convenience. In some examples, the entire diaphragm may be made of a metallic material. 
         [0026]    In some examples, the bias voltage is provided by a battery coupled through a large bias resistor to the diaphragm plate and back plate. Other sources of bias voltage may include phantom power over the signal connection, or permanent charges in one or both of the plates, as in an electric microphone. The external bias voltage, when used, is connected to one plate of the capacitor through a circuit element, such as a field-effect transistor (FET). This circuitry serves to convert the high output impedance of the capacitive sensor into a low output impedance driver, in some cases with amplification as well, for driving external circuitry with reduced susceptibility to noise and interference. Such circuitry can be generally referred as an impedance buffer. One example of an impedance buffer useful in this situation is a FET connected as a common-source amplifier. The FET provides a high input impedance so that it does not load the sensor, and also provides a low output impedance for communicating the small measured voltages between the plates along long signal lines. In addition, the FET works well because its gate leakage provides high impedance required by the bias voltage without requiring a large bias resistor as in a condenser microphone. Various types of field-effect transistors may be used, such as junction field-effect transistors (JFET) or metal-oxide field-effect transistors (MOSFET). Other types of amplifiers can also be used for the impedance buffer, and references to a FET herein is exemplary only—it is not intended to be limiting. 
         [0027]    The bias voltage and FET are coupled to the plates and provide an output signal as shown in  FIG. 5 . In this representative circuit  300 , one side of the bias supply  302  is connected to the source terminal of the FET  106  at a node  304 , and biases the stationary plate through the FET&#39;s leakage impedance. In the example of  FIG. 5 , this connection also includes a current limiting resistor  322 . This optional resistor  322  acts as a fuse to prevent the bias supply from damaging the FET if the two plates of the sensor are shorted. The other side of the bias supply  302  is connected to the front plate on the diaphragm, represented as one terminal of a variable capacitor  306 , at a node  308 . In some examples, the diaphragm and the bias supply are coupled through a common ground, rather than through a specific node  308 . The polarity of the bias supply may be reversed from that shown in  FIG. 5 . In the example that one or both of the plates is permanently charged, the bias supply  302  is essentially integrated into the plate itself (i.e., into the variable capacitor  306 ), and the battery  302  in the circuit diagram of  FIG. 5  is replaced by a virtual high-impedance source. The other terminal of the capacitor, representing the back plate located on the limiter, is coupled at node  310  to the gate terminal of the FET  106 . 
         [0028]    The capacitance of the two parallel plates is found from the well-known formula: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0000]    where k is a unitless factor to account for edge effects and is typically ≈1, ε is the dielectric constant in air (8.9×10 15  F/mm), A is the area of the plates in mm 2 , and d 0  is the resting distance between the plates in mm (any suitable system of units may be used). When the moving plate moves by an amount Δd, the capacitance becomes 
         [0000]    
       
         
           
             
               
                 
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         [0029]    From (2), capacitance is non-linearly related to the displacement. To obtain a measurement that is linearly related to displacement, a bias voltage e 0  is applied, as in a condenser microphone. A bias voltage applied across the parallel plates through a high impedance results in a constant charge Q 0  on the plates, given by: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0030]    When the moving plate moves by Δd, the charge Q 0  resulting from the bias voltage stays the same, but the voltage across the plates changes to e=e 0 +Δe, thus 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    Solving for the relationship between voltage change and displacement gives: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    Thus, as the capacitance varies, with a bias voltage applied to the plates, the voltage at the gate of the FET varies linearly with the displacement of the diaphragm. 
         [0031]    Such a linear output may be more useful in subsequent uses of the displacement measurement than an inversely-proportional measurement of actual capacitance would be. Once displacement is known, its derivative may be used to provide the velocity of the diaphragm, and that may in turn be derived into acceleration, depending on the signal processing needs of the device and its users. 
         [0032]    The FET amplifies the gate voltage to provide a corresponding voltage across the source and drain. The source terminal and drain terminal of the FET are coupled to the signal outputs  314 ,  316 , as well as to several additional components, at nodes  304  and  312 . A small capacitor  318 , on the order of 40-50 pF, across the FET from drain to source provides protection from radio-frequency (RF) noise. A small shunt capacitor  320 , on the order of 0.1 μF, provides protection from noise pickup by the bias supply connections. As mentioned above, the optional resistor  322  in series with the bias supply, on the order of 100 kΩ, protects the high-voltage bias supply from short-circuiting. Phantom power for the FET is represented by a supply  324  and bias resistor  326  across the output lines. 
         [0033]    Contrary to what might be expected, the curvature of the plates does not compromise the measurement or require more-complicated processing. While the curvature does make the measurement less linear, as the fact that the two plates remain parallel, that is, the variable distance between the plates is the same at every point, the sensor remains sufficiently linear for practical purposes. The curvature, by increasing the surface area, also helps maximize the total capacitance for a given maximum displacement. The curvature of the dome is desirable because it increases the diaphragm&#39;s stiffness and thereby reduces breakup of the diaphragm, which could lead to non-linear performance of both the transducer and the sensor. 
         [0034]    Various types of external sensor may interfere with the dynamic performance of the transducer, either by accidentally contacting the diaphragm if located too close, or by mass-loading the diaphragm if attached to it. Because the sensor described herein is integrated into the transducer, and the added moving mass of metalizing the diaphragm is negligible, it does not change the dynamic behavior of the transducer in any measurable way, leaving the acoustic performance of the transducer unchanged. An integrated sensor also works over a broad frequency range with low noise, as the body of the driver shields the sensor, both physically and magnetically, and provides an intimate connection between the sensor and the diaphragm it is measuring. Integrating the electronics allows the connections between the sensor and the FET to be very short, reducing interference from outside noise. Integration also allows the FET to amplify the signal before it ever leaves the transducer, providing a large output signal voltage that is less susceptible to noise in the signal path. The cost of adding such a sensor to a transducer may also be lower than other sensors. 
         [0035]      100351  In another embodiment, as shown in  FIGS. 6A ,  6 B, and  7 , one of the plates may be split into two electrically isolated parts  402   a  and  404   b  ( FIG. 6A ) or  02   b  and  404   b  ( FIG. 6B ), with the DC bias applied between those two parts. The other plate  406   a  ( FIG. 6A ) or  40   b  ( FIG. 6B ) is not connected to the electrical circuit except through capacitance with the two halves of the split plate. In this embodiment, the capacitances between each of the two split plates  402   a/b  and  404   a/b  and the solid plate  406   a/b  vary together. A differential voltage is created by applying opposite bias charge on the two parts of the split plate, obviating the need to connect a voltage source or signal connection to the single plate. In the case of  FIG. 6A , this has the advantage of not requiring connections to the moving diaphragm. In the alternative case of  FIG. 6B , the advantage is in not requiring circuitry inside the transducer, as the connections from the split plates  402   b  and  404   b  to the circuit board  108  may be made externally. Those connections may be made to the diaphragm in the same manner discussed above, e.g., metalizing the entire surface of the diaphragm (but for a space between the two plates) and contacting the metallized layer where it is anchored at the edge, as in  FIG. 2 . 
         [0036]    This split sensor is represented by two variable capacitors  412 ,  414  in series. The parts of the capacitor symbols corresponding to plates  402 ,  404 , and  406  are also labeled in  FIG. 7  for convenience. These two capacitors  412 ,  414  each have one-half the capacitance of a single-plate version with the same total surface area, and their capacitances change with displacement at the same rate. Each half is biased with half of the single-plate bias voltage, but the voltage change from the two halves adds, so the sensitivity to displacement remains the same as that found in equation (5) above in the single-plate version. This means that a measurement circuit  400  that is basically the same as the circuit  300  used for the single-plate version, shown in  FIG. 5 , can be used with the two-plate version, as shown in  FIG. 7 . The only structural difference is the two variable capacitors  412  and  414  representing the capacitive plates, though of course the values of specific elements may be different. The two-plate implementation also improves common-mode rejection. Because the two split plates  402  and  404  receive the same common-mode noise but are opposed to each other in the circuit  400 , that noise is isolated from the net signal output. While the split plates are shown as identical half-moon shapes in the embodiments of both  FIG. 6A  and  FIG. 6B , other shapes are possible. For example, they could be concentric rings, or a ring and a disc. It is preferable that they be of the same area, but adjustments for differing areas could be made in the circuitry. In some examples, a larger number of plates may be used, such as pie-piece segments, with every other segment having one voltage level and the other segments the opposite voltage. This may further enhance resistance to some types of noise, and would be easier to connect to external circuitry, in the case of the split plates being on the diaphragm, than concentric plates. 
         [0037]    In the split-plate embodiment shown in  FIG. 6A , all of the electrically-connected parts of the sensor are stationary, improving its mechanical reliability. The sensor components are not subject to mechanical fatigue, there is less moving mass, and the moving parts may have better balance and symmetry. Even in the embodiment of  FIG. 6B , this advantage is achieved if the connections are made at or beyond the point that the diaphragm is anchored to the basket. Avoiding additional connections to the moving parts also makes the transducer easier to assemble. This all leads to a more accurate sensor and a more robust part. 
         [0038]    The split-plate embodiment is particularly useful in certain types of transducers, such as compression drivers, where the diaphragm may already be metal, but be difficult to connect to electrically. In a compression driver, the back plate may be formed as a metallized layer on the top surface of the phase plug. A novel type of compression driver is described in U.S. patent application Ser. No. 12/490,463, filed Jun. 24, 2009, and incorporated fully here by reference. The phase plug and part of the surrounding structure and diaphragm from that application is shown combined with the present invention in  FIG. 8 . 
         [0039]    As shown in  FIG. 8 , the compression driver  500  includes a phase plug  502  and a diaphragm  504  (shown raised from its normal position). The compression driver&#39;s mechanical and electrical structures are not specifically shown. Part of the phase plug, the bridge  506 , is metallized or made of a separate, metal, part, and provides the two halves  508 ,  510  of the back plate. In the example of  FIG. 8 , the back plates are connected to the biasing and amplifying circuit (not shown) by connections  512  through the body  514  of the motor structure. These connections may be routed, for example, through legs (not shown) supporting the bridge element  506 . In other examples, the circuit may be attached to or embedded in the phase plug and may be closer to the back plate, as in the previous examples. 
         [0040]    In another embodiment, the fixed plate may be a porous screen located in front of and conformal to the diaphragm, locating the sensor on the outside of the transducer. In this example, the porosity of the fixed plate avoids significantly changing the acoustic loading on the diaphragm. This construction also avoids having to provide an electrical connection through the motor structure. 
         [0041]    In another embodiment, as shown in  FIG. 9 , a sensor  1000  is implemented as plates  1002  and  1004  located on the side of the motor structure, such that one plate slides past the other, varying the surface area of the capacitor. In this embodiment, the capacitance varies linearly with the aligned surface area and therefore with the displacement: 
         [0000]    
       
         
           
             
               
                 
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         [0000]    where w and l are the width and length of the plates, A 0 =w×l. Displacement is represented by Δl, as the moving plate moves in the direction of the length, while the gap d 0  remains unchanged. To convert the capacitance to an output signal representing the displacement, the electrodes are coupled to an RF bridge circuit  1006  such as that shown in  FIG. 10 . The circuit board  1008  may be located on the outside of the motor structure, and coupled to the plate(s) by leads  1012  (no lead to the moving plate is shown). As in the example of  FIG. 6 , there may be two plates on the stationary side, removing the need to provide a connection to the moving plate  1002 . In the RF bridge circuit  1006 , the capacitor formed by the moving plates is represented as a variable capacitor  1010 . Fixed capacitor  1012  and resistors  1014  and  1016  cause the voltage at the center of the bridge, amplified by an amplifier  1018 , to vary with the capacitance of the plates. A bias supply  1020  provides the fixed charge to the plates. If two plates are used on the stationary side, they are represented in the circuit by two capacitors back-to-back as in the example  FIG. 7 . 
         [0042]    Electromechanical transducers include electroacoustic transducers (also referred to as loudspeakers and microphones), linear or rotary electric motors, and electromechanical sensors. This disclosure is concerned generally with transducers that cause or measure small and generally oscillating movements, where a moving portion of the transducer moves back and forth around a stationary portion. For example, in a loudspeaker, the acoustically-radiating surface, referred to as the diaphragm, and some portion of the motor structure move back and forth, while another portion of the motor structure remains stationary. In some examples, the moving portion of the motor is a voice coil positioned around a magnetic structure. In other examples, the voice coil is inside a hollow magnetic structure. In still other examples, the coil is stationary and it is the magnet that moves the diaphragm, or the diaphragm is magnetically responsive and requires no additional moving components. In non-acoustic applications, an electromagnetic linear motor includes a moving armature and a stationary stator. Either one of the armature and stator may include the magnets and the other the coils or some other mechanism for converting electric energy into motion of the armature. 
         [0043]    Other implementations are within the scope of the following claims and other claims to which the applicant may be entitled.