Patent Publication Number: US-2020289013-A1

Title: Miniature electric field detector

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 62/819,222, titled “ENHANCED DIAGNOSTICS USING 3D CARDIAC SENSING WITHOUT ELECTRODES AND LEADS,” filed on Mar. 15, 2019, which is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The human body generates static and time-varying electromagnetic fields which may be measured and used in numerous applications. However, these fields are often faint, even in close proximity to the body, and attenuate as the distance from the human body is increased. For example, ionic currents within muscles of the human body, such as the heart and skeletal muscles (for example, calves, quadriceps, and so forth), will generate voltage fluctuations and magnetic fields during synaptic transmission. While these fields have proven challenging to accurately measure, some approaches exist for directly detecting the electrical activity produced by the body. For example, to determine electromagnetic activity of a patient&#39;s heart, numerous electrodes are arranged to measure scalar potential differences across a patient&#39;s chest with an electrocardiogram (ECG). A vectorcardiogram (VCG), which may be generated based on multiple ECG measurements, is a 3D vector representation of the patient&#39;s heart&#39;s electric field, estimated based on the ECG measurements. Electromagnetic activity of the patient&#39;s heart may be determined based on the VCG. 
     SUMMARY 
     Aspects and examples discussed herein include a sensor system comprising a first substrate configured to be coupled to a user, an electric field detector to detect an electric field generated by the user, the electric field detector being coupled to the first substrate and comprising a second substrate, a proof mass positioned above the second substrate, one or more electrodes coupled to the second substrate, and a control circuit coupled to the one or more electrodes, the control circuit being configured to determine a respective change in capacitance between the proof mass and each respective electrode of the one or more electrodes responsive to torsional movement of the proof mass in response to the electric field, and a controller coupled to the first substrate and to the electric field detector, the controller being configured to receive, from the electric field detector, information indicative of each respective change in capacitance between the proof mass and each respective electrode of the one or more electrodes, and determine, based on the information indicative of each respective change in capacitance between the proof mass and each respective electrode, characteristics of the electric field in at least two dimensions. 
     In some examples, the electric field detector is removably coupled to the first substrate. In various examples, the system further comprises an adhesive coupled to the first substrate, the first substrate being configured to be removably coupled to the user. In at least one example, the sensor system further comprises an electric dipole coupled to the proof mass, the electric dipole being polarized along a polarization axis. In some examples, the proof mass is configured to rotate about a first torque axis orthogonal to the polarization axis responsive to the electric field having a first vector component aligned with a first electric field axis, the first electric field axis being orthogonal to the polarization axis and the first torque axis, and rotate about a second torque axis orthogonal to the polarization axis responsive to the electric field having a second vector component aligned with a second electric field axis, the second electric field axis being orthogonal to the polarization axis and the second torque axis, wherein the second torque axis is parallel to the first electric field axis and the first torque axis is parallel to the second electric field axis. 
     In various examples, the one or more electrodes includes a first set of one or more electrodes and a second set of one or more electrodes, the control circuit being configured to determine a first change in capacitance between the proof mass and the first set of one or more electrodes responsive to torsional movement of the proof mass about the first torque axis, and determine a second change in capacitance between the proof mass and the second set of one or more electrodes responsive to torsional movement of the proof mass about the second torque axis. In at least one example, the controller is further configured to determine, based on the first change in capacitance and the second change in capacitance, characteristics of the electric field along the first electric field axis and the second electric field axis. 
     In some examples, the electric dipole includes a dielectric material, and wherein the control circuit is configured to selectively polarize the dielectric material along a first polarization axis and a second polarization axis, the first polarization axis being orthogonal to the second polarization axis. In at least one example, the proof mass is configured to rotate about a first torque axis orthogonal to the first polarization axis responsive to receiving the electric field along a first electric field axis, the first electric field axis being orthogonal to the first polarization axis and the first torque axis, rotate about a second torque axis orthogonal to the first polarization axis responsive to receiving the electric field along a second electric field axis, the second electric field axis being orthogonal to the first polarization axis and the second torque axis, and rotate about a third torque axis orthogonal to the second polarization axis responsive to receiving the electric field along a third electric field axis, the third electric field axis being orthogonal to the second polarization axis and the third torque axis, wherein the first torque axis is parallel to the second electric field axis and one of the third electric field axis and the second polarization axis, the second torque axis is parallel to the first electric field axis and one of the third electric field axis and the second polarization axis, and the third torque axis is parallel to the first polarization axis. 
     In at least one example, the one or more electrodes includes a first set of one or more electrodes, a second set of one or more electrodes, and a third set of one or more electrodes, the control circuit being configured to determine a first change in capacitance between the proof mass and the first set of one or more electrodes responsive to torsional movement of the proof mass about the first torque axis, determine a second change in capacitance between the proof mass and the second set of one or more electrodes responsive to torsional movement of the proof mass about the second torque axis, and determine a third change in capacitance between the proof mass and the third set of one or more electrodes responsive to torsional movement of the proof mass about the third torque axis. In some examples, the controller is further configured to determine, based on the first change in capacitance, the second change in capacitance, and the third change in capacitance, characteristics of the electric field along the first electric field axis, the second electric field axis, and the third electric field axis. 
     In various examples, further comprising a first set of polarization electrodes and a second set of polarization electrodes coupled to the dielectric material, the first set of polarization electrodes being positioned along the first polarization axis and the second set of polarization electrodes being positioned along the second polarization axis. In some examples, the control circuit is configured to generate a first voltage difference across the first set of polarization electrodes to polarize the dielectric material along the first polarization axis, and generate a second voltage difference across the second set of polarization electrodes to polarize the dielectric material along the second polarization axis. In at least one example, generating the first voltage difference includes applying a first voltage to the first set of polarization electrodes at a first frequency, and wherein generating the second voltage difference includes applying a second voltage to the second set of polarization electrodes at a second frequency, the first frequency being different than the second frequency. 
     In some examples, the electric field detector is configured to detect an electric field generated by a muscle of the user. In various examples, the controller is configured to determine characteristics of an electric field generated by a heart of the user. In at least one example, the controller is configured to determine characteristics of the electric field in three orthogonal dimensions. In some examples, the sensor system further comprises a movement sensor configured to determine information indicative of movement of the electric field detector, the controller being coupled to the movement sensor and being configured to receive the information indicative of the movement of the electric field detector, and determine the characteristics of the electric field based on the information indicative of each respective change in the capacitance between the proof mass and each respective electrode of the one or more electrodes and the information indicative of the movement of the electric field detector. In various examples, determining the characteristics of the electric field based on the information indicative of each respective change in the capacitance between the proof mass and each respective electrode of the one or more electrodes and the information indicative of the movement of the electric field detector includes identifying motion artifacts caused by the movement of the electric field detector. 
     According to another aspect discussed herein, an electric field detector to detect an electric field generated by a user is provided, the electric field detector comprising a substrate, a proof mass positioned above the substrate, a plurality of electrodes coupled to the substrate, the plurality of electrodes including a first set of one or more electrodes and a second set of one or more electrodes, and a control circuit coupled to the electrode, the control circuit being configured to determine a first change in capacitance between the proof mass and the first set of one or more electrodes responsive to torsional movement of the proof mass about a first torque axis and to determine a second change in capacitance between the proof mass and the second set of one or more electrodes responsive to torsional movement of the proof mass about a second torque axis orthogonal to the first torque axis in response to being exposed to the electric field generated by the user. 
     Still other aspects, embodiments, and advantages of these exemplary aspects and embodiments, are discussed in detail below. Moreover, it is to be understood that both the foregoing information and the following detailed description are merely illustrative examples of various aspects and embodiments, and are intended to provide an overview or framework for understanding the nature and character of the claimed aspects and embodiments. Any embodiment disclosed herein may be combined with any other embodiment in any manner consistent with at least one of the objectives, aims, and needs disclosed herein, and references to “an embodiment,” “some embodiments,” “an alternate embodiment,” “various embodiments,” “one embodiment” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment. Various aspects, embodiments, and implementations discussed herein may include means for performing any of the recited features or functions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the disclosure. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures: 
         FIG. 1  is a chart showing examples of desirable performance metrics for a compact electric field detector; 
         FIG. 2A  is a perspective view of an electric field detector, shown with a housing detached from the detector, according to examples discussed herein; 
         FIG. 2B  is perspective view of the electric field detector illustrated in  FIG. 2A  with the housing attached, according to examples discussed herein; 
         FIG. 3  is another perspective view of components of the electric field detector illustrated in  FIG. 2A , according to examples discussed herein; 
         FIG. 4  is a perspective view of an array of electric field detectors incorporated within a headset, according to examples discussed herein; 
         FIG. 5  is a plan view of examples of sense electrodes and drive electrodes of an example of the electric field detector illustrated in  FIG. 2A , according to examples discussed herein; 
         FIG. 6  is a block diagram of a control circuit according to examples discussed herein; 
         FIG. 7A-7C  is a process flow for fabricating an example of an electric field detector, according to examples discussed herein; 
         FIGS. 8A-8C  show a state of an electric field detector during each act of the process flow of  FIG. 7A-7C , according to examples discussed herein; 
         FIG. 9  is an axial view of a proof mass and levitation forcers, according to various examples discussed herein; 
         FIG. 10  is a side profile view of a levitation suspension system including the levitation forcers of  FIG. 9 , according to various examples discussed herein; 
         FIG. 11A  illustrates a side cross-sectional view of an electric field detector according to an example; 
         FIG. 11B  illustrates a top view of an electric field detector according to an example; 
         FIG. 12  illustrates a block diagram of a sensor system according to an example; 
         FIG. 13  illustrates a block diagram of a distributed sensor system according to an example; 
         FIG. 14  illustrates a perspective view of an electric field detector according to an example; and 
         FIG. 15  illustrates a perspective view of an electric field detector according to an example. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects and embodiments are generally directed to detectors for exploiting the electric component of electromagnetic signals. Particular examples may include an electric field detector capable of detecting bio-physical signals generated by the body of a patient or user, such as the electric field of his or her muscles, including the patient&#39;s heart or skeletal muscles. Other examples of the electric field detector described herein may be suitable for detecting other weak electromagnetic signals. 
     In one example, the electric field detector is a microelectromechanical-system-based (MEMS-based) electric field detector which measures one or more torques on a suspended proof mass to determine one or more characteristics of a received electric field. In particular, an electric dipole is generated on the proof mass by placing a quasi-permanently charged material, such as a polymer electret, on the proof mass. In another example, an electric dipole is generated on the proof mass by temporarily charging a dielectric material coupled to the proof mass with an applied voltage along one or more axes, to selectively generate an electric dipole. In either example, the electric dipole generates a torque on the proof mass when exposed to an external electric field in certain dimensions. The torque induces torsional motion in the proof mass, which causes a capacitance between one or more sense electrodes and the proof mass to change. The change in capacitance may then be measured to estimate one or more characteristics of the external electric field, such as a direction, phase, and/or a magnitude. As used herein, “aspects of an electric field,” “characteristics of an electric field,” “parameters of an electric field,” and so forth, may refer to a direction, phase, and/or magnitude of an electric field. 
     In one example, the electric field detector may be integrated with one or more additional components (including, for example, an energy storage device, a controller, power conditioning circuitry, a communication interface, and so forth) in a single unit capable of determining an electrical field generated by a patient&#39;s body. For example, the electric field detector may be integrated into, or removably coupled to, an adhesive patch which can be adhered to a patient&#39;s body. Once connected to a patient, the electric field detector may detect an electric field generated by a muscle proximate to the location on the patient&#39;s body to which the adhesive patch is adhered. For example, the adhesive patch may be adhered to a patient&#39;s chest to detect electrical fields generated by the patient&#39;s heart, or may be adhered to a patient&#39;s legs to detect electrical fields generated by the patient&#39;s calves and/quadriceps, or may be adhered to any other portion of a patient&#39;s body to detect electrical fields generated by other muscles. In other examples, the electric field detector may be integrated into another package to be disposed proximate to a patient&#39;s body, such as a patient&#39;s clothing, a compressive band, a watch band, compressive straps, and so forth. In still other examples, the electric field detector may be integrated into a catheter system or implantable device. For example, the electric field detector may be integrated into a catheter system to measure intracardiac signals produced by a patient&#39;s heart. 
     In some examples, the electric field detector may include multiple elements. For example, the electric field detector may include multiple single-axis elements, each configured to determine characteristics of an electric field in a respective dimension. The multiple single-axis elements may be arranged to measure the strength of an electric field in two orthogonal dimensions or in three orthogonal dimensions. In other examples, the electric field detector may include one or more multi-axis elements, which may be integrated into a monolithic structure, and configured to determine characteristics of an electric field in multiple dimensions, which may be orthogonal dimensions. 
     One performance metric for sensors configured to detect electrical fields generated by a biological source, such as the heart, brain, or skeletal muscles, includes a noise-performance-versus-volume. For example, various sources have discussed the use of electric field encephalography (EFEG) to estimate brain activity. In particular, some literature has estimated a strength of the relevant bio-electrical signals generated by the brain. Based on the estimated strength of the relevant signals, the performance requirements for an electric field detector capable of detecting these bio-electrical signals can be determined.  FIG. 1  illustrates a graph  100  of an example of the performance requirements (for example, noise-performance-versus-volume) for one such electric field detector. In particular,  FIG. 1  illustrates these performance requirements (for example, area  102 ) relative to the performance capabilities of currently available technology (for example points  104 ).  FIG. 1  illustrates that the predicted signal magnitudes of the relevant bio-electrical signals are below the noise floor of current electric field sensors (for example, mechanical, optical, and electrical-based sensors) that could be made compact and inexpensive enough for use in diagnostic applications. 
     Accordingly, various aspects and examples discussed herein are capable of meeting the performance requirements  102  illustrated in  FIG. 1 . That is, the electric field detector described herein is capable of directly measuring bio-electrical signals, such as brain activity or muscular activity, with an improvement in signal-to-noise ratio and volume. In some instances, the electric field detector is capable of meeting these performance requirements without contacting the head or body of the given patient or user. Such a design offers the benefit of improved user comfort and convenience. While described herein primarily in the context of bio-electrical signals, it is appreciated that various examples of the electric field detector described herein may also offer significant advantages in other areas of electric field detection. 
     It is to be appreciated that examples and/or embodiments of the apparatus and methods discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The apparatus and methods are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, elements and features discussed in connection with any one or more examples and embodiments are not intended to be excluded from a similar role in any other example or embodiment. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. Any references to front and back, left and right, top and bottom, upper and lower, above and below, and vertical and horizontal are intended for convenience of description, not to limit the present systems and methods or their components to any one positional or spatial orientation. 
     The accompanying drawings are included to provide illustration and a further understanding of the various aspects and examples, and are incorporated in and constitute a part of this disclosure. The drawings, together with the remainder of the disclosure, serve to explain principles and operations of the described and claimed aspects and examples. 
       FIGS. 2A and 2B  each illustrate a perspective view of an electric field detector  200  according to various examples described herein.  FIG. 2A  illustrates a view of the detector  200  with a housing  210  detached from the detector  200 , and  FIG. 2B  shows a view of the detector  200  with the housing  210  attached. The housing  210  may be removed in a vertical direction away from the detector  200  (for example, direction  224 ), as shown in  FIG. 2A . In  FIGS. 2A and 2B , the electric field detector  200  includes a MEMS-based resonator, which may be defined by processing a structure wafer (for example, a silicon-on-insulator [SOI] wafer) to a desired geometry. As shown, the detector  200  may include a proof mass  202  coupled to a source of concentrated charge  204 , a plurality of supports  206   a ,  206   b  (collectively “supports  206 ”), one or more flux concentrators  208   a ,  208   b  (collectively “flux concentrators  208 ”), the housing  210 , one or more anchors  212   a ,  212   b  (collectively “anchors  212 ”), a baseplate  214 , one or more electrical contacts  216 , one or more leads  218 , and a substrate  222 , among other components. While not shown in  FIGS. 2A and 2B , each of the contacts  216  may couple the electric field detector  200  to a control circuit, examples of which are further discussed herein. In certain examples, the structure wafer is processed (for example, etched) to define the proof mass  202 , the plurality of supports  206 , and the one or more anchors  212 . In further examples, the electric field detector  200  may also include one or more counterbalances  226  that are coupled to the proof mass  202 . In certain examples, the electric field detector  200  may also include one or more sense electrodes and one or more drive electrodes, each of which are positioned on the substrate  222  and obscured in  FIGS. 2A and 2B  by the counterbalance  226 . As shown, the substrate  222  is positioned on the baseplate  214   
     In various examples, the electric field detector  200  determines one or more characteristics of a received electric field, which in one instance is a bio-electrical signal, based on measured capacitance variations due to torsional motion of the proof mass  202  in response to receiving the electric field. While in some examples, a combination of linear forces may result in the torsional motion of the proof mass  202 , in certain other examples, a variation in capacitance as a result of a single linear force may be measured. The proof mass  202  is supported by the plurality of supports  206 , each of which form a rotationally compliant spring anchored to the substrate  222  via a respective anchor  212   a ,  212   b . In the shown example, each support  206  is a flexured beam interposed between a side surface of the proof mass  202  and a corresponding anchor  212   a ,  212   b . That is, a first support  206   a  is interposed between a first side surface of the proof mass  202  and a first anchor  212   a , and a second support  206   b  is interposed between a second side surface of the proof mass  202  and a second anchor  212   b . Each anchor is coupled to the substrate  222  with a respective anchor ground  220   a ,  220   b . The first anchor  212   a  is coupled to the substrate  222  at the first anchor ground  220   a , and the second anchor  212   b  is coupled to the substrate  22  at the second anchor ground  220   b.    
     As shown in  FIG. 2A , the first support  206   a  and the second support  206   b  may be coupled to opposing sides of the proof mass  202 . The dimensions of the supports  206  are selected such that the overall stiffness of the supports  206  are sufficient to withstand operational shock loads while maximizing a response to input torques. While shown as including a pair of supports  206   a ,  206   b , in various other examples the electric field detector may include one (for example, in a “lever” arrangement) or any number of supports  206 . For instance, the detector  200  may include three supports  206 , or an arrangement of four or more supports  206 . 
     In various other examples, the proof mass  202  may be levitated by an electrostatic suspension, levitated by an electromagnetic suspension, and/or suspended by an equivalent rotational bearing. Unlike the example illustrated in  FIG. 2A , in these examples it may be advantageous to design the proof mass  202  (and/or source of concentrated charge  204 ) to have a circular or cylindrical shape to permit rotation thereof. In such an example, the levitated proof mass (for example, relative to a substrate) is positioned to move (for example, rotate) with very low resistance and low stiffness. Such an arrangement may maximize a scale factor of the electric field detector  200  while retaining a structural stability and robustness. In such an example, the electrostatic suspension, electromagnetic suspension, and/or rotational bearing may supplement the one or more illustrated flexured beams of  FIG. 1  (for example, supports  206 ) or replace the one or more flexured beams. 
     One example of a levitation suspension system  1000  is described with reference to  FIG. 9  and  FIG. 10 . In particular,  FIG. 9  illustrates an axial view of a proof mass  902  and levitation forcers  904 , and  FIG. 10  illustrates a profile view of a levitation suspension system  1000  that includes the levitation forcers  904  of  FIG. 9 . Examples of the levitation suspension system  1000  may be incorporated within any of the examples of the electric field detectors described herein, such as the electric field detector  200  described with reference to  FIG. 2A  and  FIG. 2B . That is, the proof mass  902  may be the proof mass  202  illustrated in  FIG. 1 .  FIG. 9  illustrates an axial view of a proof mass  902  and levitation forcers  904 , and  FIG. 10  shows a side profile view of the levitation suspension system  1000 . As shown, the levitation suspension system  1000  may include one or more levitation forcers  904  that apply a levitating force to the proof mass  902  to levitate the proof mass against gravity and other induced forces. In certain examples, each of the one or more levitation forcers  904  may include one or more sense electrodes  502  or drive electrodes  504  further described below with reference to  FIG. 5 . While in certain examples, each levitation forcer  904  may be an electrostatic forcer (for example, for electrostatic levitation), in various other examples, each levitation forcer  904  may be a magnetic forcer (for example, for magnetic levitation). 
     A control circuit  1002  (for example, control circuit  600  illustrated in  FIG. 6 ) coupled to the levitation forcers  904  receives feedback from each levitation forcer  904  and/or one or more feedback sensors  1004 . If a position of the proof mass  902  is displaced relative to a desired null point (for example, shown as point  1006 ), the control circuit  1002  provides a control signal to one or more of the levitation forcers  904  to increase or decrease the force applied by the receiving levitation forcer  904  and return the proof mass  902  to the null position. In certain examples, the proof mass  902  may be metalized (for example, at an end of the proof mass) to increase the sensitivity of the proof mass  902  to the levitation force. The position of the proof mass  902  (relative to the null position) may be capacitively measured based on a capacitance between the proof mass  902  and one or more sense electrodes (for example, sense electrodes  502  described with reference to  FIG. 5 ). 
     The number and arrangement of levitation forcers  904  may be selected based on the desired application of the corresponding electric field detector. While  FIG. 9  illustrates a plurality of levitation forcers  904  (for example, four) radially aligned about the circumference of an axial proof mass  902 , various other arrangements are possible. In particular, the number, shape, and arrangement of levitation forcers  904  may depend on the particular shape of the proof mass  902  and packaging constraints (for example, size, weight, available space, etc.). In addition to maintaining the proof mass  902  a desired null position, in certain instances, the levitation forcers  904  may be used to rotate the proof mass  902  at a desired velocity, or reposition the proof mass  902  to a desired orientation. In addition to assessing the position of the proof mass  902  relative to a null position, one or more signals from the illustrated feedback sensor  1004  may be used by the control circuit  1002  to infer external stimuli that induce proof mass  902  movement. The feedback sensor  1004  may be an optical sensor, an accelerometer, a capacitive sensor, or any other type of position sensor. 
     Referring to  FIGS. 2A and 2B , in various examples, the plurality of supports  206  may suspend the proof mass  202  above a substrate offset space defined in the substrate  222 . That is, the substrate  222  may include an area (referred to as a “substrate offset space”) formed in a surface thereof beneath the proof mass  202  (for example, and counterbalance  226  shown in  FIGS. 2A and 2B ). The substrate offset space is obscured in  FIGS. 2A and 2B  by the counterbalance  226 . While described as being suspended “above” the substrate offset space, in other examples, the proof mass  222  may be partially positioned within the substrate offset space. In other examples, the proof mass  202  may be positioned in close proximity to the substrate offset space but not directly above the substrate offset space. As discussed, in certain examples, the electric field detector  200  may include one or more sense electrodes and one or more drive electrodes, each of which are positioned on the substrate  222  and in capacitive communication with the proof mass  202 . In particular, each of the sense electrodes and the drive electrodes may be positioned within the substrate offset space and may form a sense gap with the proof mass  202 . In certain examples, the substrate offset space is formed by etching the substrate  222 ; however, other processing techniques may be used to form the substrate offset space, such as milling, grinding, or one or more deposition processes. Various aspects of a substrate, a substrate offset space, sense electrodes, and drive electrodes are discussed below with reference to at least  FIG. 7A-7C  and  FIGS. 8A-8C . 
     In various examples an impinging electric field concentrated on the source of concentrated charge  204  generates a torque and effects motion of the proof mass  202 . For instance, the torque, τ, may be represented as: 
       τ= p×E  
 
     where p is the strength of the electric dipole from the source of concentrated charge  204  (for example, in C-m) and E is the strength of the received electric field (for example, in V/m). 
     In many instances, the proof mass  202  responds to the torque by rotating about a torque axis. In one example, the rotation can be represented as: 
     
       
         
           
             θ 
             = 
             
               τ 
               
                 
                   ( 
                   
                     Is 
                     2 
                   
                   ) 
                 
                 + 
                 
                   ( 
                   Ds 
                   ) 
                 
                 + 
                 k 
               
             
           
         
       
     
     where θ is the angle of rotation, τ is the torque, I is the polar moment of inertia, s is the complex frequency, D is a damping coefficient, and k is the rotational stiffness. In this way, the torque generated from the electric field induces motion in the proof mass  202 , which reacts against the stiffness of the supports  206  (or the levitation suspension system  1000 ). 
     In some examples, the proof mass  202  may be capable of rotating about multiple torque axes. For example,  FIGS. 2A, 2B, and 3  illustrate a first legend  232  and a second legend  234 . The legends  232 ,  234  include a first respective axis, labeled “E,” indicating a direction of an external electric field, a second respective axis, labeled “τ,” indicating an axis about which the proof mass  202  rotates in response to the external electric field, and a third respective axis, labeled “p,” indicating a direction of polarization of the source of concentrated charge  204 . 
     Although the supports  206  may be particularly well-suited for rotating about the torque axis τ indicated by the first legend  232 , the supports  206  may be sufficiently flexible that the proof mass  202  can rotate about the torque axis τ indicated by the second legend  234  in a manner that can be detected by the electric field detector  200 . Accordingly, an electric field may be detected in at least two orthogonal dimensions by the electric field detector  200  in some examples, including the first respective axes in each of the legends  232 ,  234 . Furthermore, it is to be appreciated that the electric field detector  200  may be configured to detect an electric field along a different combination of axes by varying a polarization direction of the source of concentrated charge  204 . 
     In various examples, the rotation of the proof mass  202  increases or decreases the distance between the proof mass  202  and the sense electrode(s) positioned on the substrate  222 . In examples in which the electric field detector  200  is configured to detect an electric field in multiple (for example, two) dimensions, there may be multiple sets of one or more sense electrode(s) positioned on the substrate  222 , each set being configured to detect increases or decreases in distance between the proof mass  202  and the sense electrode(s) caused by a different component of the electric field. As the distance between the proof mass  202  and the sense electrode(s) increases or decreases, the relative capacitance between the sense electrode(s) and the proof mass  202  varies. The resulting change in capacitance can be measured by the electronics to estimate the characteristics of the received electric field. In various examples, the electric field detector  200  may include a plurality of electrical leads  218 , at least one of which couples a sense electrode to a corresponding contact  216 . Each electrical contact  216  may connect the corresponding lead  218  to the control circuit, which may determine a direction (or directions), a magnitude, and/or a phase of the received electric field based on the sensed variation in capacitance. For example, the control circuit may determine a direction, magnitude, and/or phase of a received electric field based on the sensed variation in capacitance from a first set of one or more sense electrodes, and may determine a direction, magnitude, and/or phase of the received electric field based on the sensed variation in capacitance from a second set of one or more sense electrodes. As illustrated, the substrate  222  may be coupled to the baseplate  214 . Accordingly, the baseplate  214  supports the substrate  222 , as well as other components of the detector  200 , and may include one or more fasteners for creating a seal with the housing  210 . 
     In certain examples, the control circuit may also send one or more control signals to the electrical contacts  216  and the corresponding leads  218 . In particular, the control circuit may generate one or more control signals which can be used charge one or more drive electrodes and produce a feedback torque on the proof mass  202 . That is, the electric field detector  200  may further include one or more drive electrodes positioned on the substrate  222  (for example, within the substrate offset space) which rebalance the proof mass  202  to a nominal rotational position based on a received control signal. Such an arrangement may reduce non-linearities in the capacitance measurements (for example, from the supports  206 ) while also extending the dynamic range of the electric field detector  200 . In such an example, a lead  218  may receive the control signal from a contact  216  and provide the control signal to a drive electrode. 
     In certain examples, the electric field detector  200  may include a source of concentrated charge  204  (for example, concentrated electrical charge). In the example shown in  FIG. 2A , the source of concentrated charge  204  is coupled to a top surface of the proof mass  202 ; however, in certain other examples, the proof mass  202  itself may be composed of charge-concentrated material. That is, a body of the proof mass  202  may be composed of a source of concentrated charge. In various examples, the source of concentrated charge  204  may include any suitable source of a semi-permanent static electric dipole, such as an electret or a capacitor plate having a residual free charge and/or polarization. As will be understood to one of ordinary skill in the art, the term “electret” refers to the dielectric equivalent of a permanent magnet. 
     For example, an electret configured for use in the detector  200  may be formed by: (a) applying heat to the electret material, (b) in response to obtaining a predetermined temperature, applying a voltage to the electret material, at which point the electret material will act like a capacitor and store the applied charge, and (c) cooling the electret material to a predetermined temperature. Thereafter, the electret maintains a residual charge after the field is removed. As an additional example, the electret material may be bombarded with radiation to generate a residual charge. Accordingly, real surface charges or aligned dipoles are immobilized in the bulk of the dielectric material. 
     Materials such as polytetrafluoroethylene, silicon nitride, fluorinated ethylene propylene, a perfluoroalkoxy alkane material, Cyptop, cyclotene, and other dielectrics may be suitable materials that can be used as an electret. In certain examples the electret may include, but is not limited to, thermo-electrets, metal-polymer electrets, radio-electrets, and mechanoelectrets. In some examples, the source of concentrated charge  204  may be charged (that is, by applying a voltage thereto) prior to coupling the source of concentrated charge  204  to the proof mass  202 . In certain other examples, the source of concentrated charge  204  may be first coupled to the proof mass  202 , and then charged. After formation, residual surface potentials can be maintained with no power input since the charge is retained in the source of concentrated charge  204  (for example, in deep traps within the electret material). In some instances, the residual surface potential may be more than 1 kV. 
     Further examples of the source of concentrated charge  204  may include a series of two or more stacked electrets or a plurality of electrets arranged in a predetermined order. To increase the strength of the electric dipole, and therefore increase the sensitivity of the detector  200  to electric fields, micron-thick layers of electrets may be stacked together. Metal layers may be interposed between one or more layers of the source of concentrated charge  204  (for example, stacked electret layers) to increase the gain of the one of more field concentrators  208  positioned adjacent the proof mass  202 . For example, the metal layers of some embodiments may include layers of gold or platinum. 
     In other examples, the source of concentrated charge  204  may generate a semi-permanent dynamic electric dipole by driving a piezoelectric material (for example, PZT). For instance, the control circuit may continuously, or periodically, drive the PZT to refresh the charge distribution when depleted. In other examples, the control circuit may actively generate a voltage gradient across the proof mass  202  of the electric field detector  200  (or a dielectric material connected thereto) to generate a dynamic electric dipole. In such an example, one or more electrodes or piezoelectric materials may supply an induced voltage (for example, active excitation signal) to vary a dynamic electric dipole at the proof mass  202 . Specifically, the electrodes may be driven by the control circuit at an alternating-current (AC) frequency such that the detector  200  up-converts (for example, increases a frequency) the received electric field information to a frequency above a 1/f noise limit, improving the performance of the detector  200 . For example, the control circuit may drive the electrodes at an AC frequency that is based on (for example, substantially equal to) a resonant frequency of the proof mass  202 . 
     In one example, a dynamic electric dipole is provided by coupling a selectively charged component to the proof mass  202 . For example,  FIG. 11A  illustrates a side cross-sectional view of a portion of an electric field detector  1100  according to an example and  FIG. 11B  illustrates a top view of a portion of the electric field detector  1100  according to an example. The electric field detector  1100  includes a proof mass  1102 , a dielectric component  1104 , a first electrode  1106   a  and a second electrode  1106   b  (collective, “electrodes  1106 ”), and a first trace  1108   a  and a second trace  1108   b  (collectively, “traces  1108 ”). The electric field detector  1100  may be substantially similar to the electric field detector  200 , except that the dielectric component  1104  provides a dynamic electric dipole in lieu of the source of concentrated charge  204 . 
     The first trace  1108   a  is coupled to the first electrode  1106   a , and is configured to be coupled to a power source. For example, the first trace  1108   a  may be coupled to a power source configured to provide a positive voltage relative to a reference voltage (for example, ground). The first electrode  1106   a  is coupled to a first surface of the dielectric component  1104 , and is configured to apply a voltage supplied by the power source to the first surface of the dielectric component  1104 . 
     The second trace  1108   b  is coupled to the second electrode  1106   b , and is configured to be coupled to a power source, which may be the same power source or a different power source than that coupled to the first trace  1108   a . For example, the second trace  1108   b  may be coupled to a power source configured to provide a negative voltage relative to the reference voltage. The second electrode  1106   b  is coupled to a second surface of the dielectric component  1104 , which may be an opposite surface from the first surface of the dielectric component  1104 , and is configured to apply a voltage (for example, an AC voltage) supplied by the power source to the second surface of the dielectric component  1104 . 
     In one example, where the first trace  1108   a  applies a positive voltage to the first electrode  1106   a  from the power source and the second trace  1108   b  applies a negative voltage to the second electrode  1106   b  from the power source, a potential difference is generated across the dielectric component  1104 . In various examples, the dielectric component  1104  may include a dielectric material or materials such that an electric dipole is generated across the dielectric component  1104 . The electric dipole generated by the dielectric component  1104  may be similar to that provided by the source of concentrated charge  204 . However, the dielectric component  1104  may be selectively and configurably charged, rather than being substantially fixedly charged. For example, the power source or power sources coupled to the traces  1108  may provide AC power to the electrodes  1106  at a configurable frequency. Moreover, in some examples, the electric field detector  1100  may include several sets of one or more electrodes positioned along various axes of the dielectric component  1104  such that the dielectric component  1104  may be selectively charged along the various axes. That is, although  FIGS. 11A and 11B  illustrate the electrodes  1106  as being positioned along one axis, in other examples, the electric field detector  1100  may be coupled to electrodes positioned along any of three axes of three-dimensional space such that the electric field detector  1100  may be polarized along any of the three axes of three-dimensional space. 
     The power source or power sources may drive the electric dipole at a carrier frequency to improve electric field sensitivity within certain bands. For example, the carrier frequency may be tuned to a resonant frequency of the dipole structure (including, for example, the proof mass  1102  and/or the dielectric component  1104 ) to improve sensitivity at that frequency. In another example, the carrier frequency may be set higher than electric field frequencies of interest (that is, the frequencies of the electric fields generated by a patient) such that the amplified signal of interest may be up-modulated to lower noise frequency bands of the amplifier. The amplified signal of interest may be subsequently demodulated following amplification. 
     As illustrated in at least  FIGS. 2A-2B , in at least one example the proof mass  202 , the supports  206 , and the anchors  212   a ,  212   b  are defined in a same structure wafer. For instance, the structure wafer may include an SOI wafer having a flexure layer, a handle layer, and an oxide layer. The oxide layer may be interposed between the flexure layer and the handle layer. As further described with reference to  FIG. 7A-7C  and  FIGS. 8A-8C , one example of the proof mass  202 , the supports  206 , and the anchors  212   a ,  212   b  may be defined in the flexure layer. It is appreciated that in some instances, the source of the concentrated charge  204  and/or an intervening material (for example, a glue or other adhesive material) between the source of concentrated charge  204  and the proof mass  202  may introduce an asymmetry in a balance of the proof mass  202 . Such an asymmetry may generate undesired sensitivities to external accelerations. In certain particular examples, the electric field detector  200  may include the one or more counterbalances, such as the counterbalance  226 , to compensate for asymmetries. 
     In various examples, the electric field detector  200  may alternatively or additionally compensate for the external accelerations, and/or effects from other external parameters, by directly measuring the external parameter with an auxiliary sensor, and adjusting the measured electric field to compensate for the external parameter. For instance, in addition to external movements and/or accelerations, the auxiliary sensor may measure at least one of noise, ambient temperature, or vibrations. Accordingly, the auxiliary sensor may include an accelerometer, temperature sensor, or noise sensor, to name a few examples. The control circuit may receive measurements from the auxiliary sensor using various filtering techniques (for example, digital signal processing filter techniques), for example, to adjust the characteristic of the electric field to compensate for the effect(s) of the measured external parameter on the measured characteristic of the electric field. In various examples, adjusting the measured characteristic of the electric field may include applying a filter to remove the effect(s) of the external parameter. For example, movement of the electric field detector  200  may cause certain undesirable motion artifacts to appear. By identifying movement of the electric field detector  200  with an auxiliary sensor, such as an accelerometer, optical sensor, or magnetic sensor, these motion artifacts may be identified and eliminated as having been caused by movement of the electric field detector  200 . The particular arrangement and position of auxiliary sensors within the electric field detector  200  may vary based on the particular external parameter desired to be measured, as well as the particular architecture of the electric field detector  200  itself. Accordingly, an auxiliary sensor is generally represented by auxiliary sensor block  230  in  FIG. 2A  (not illustrated in  FIG. 2B  and  FIG. 3 ). 
     Referring to  FIG. 3 , there is illustrated a view of the electric field detector  200  shown in  FIGS. 2A and 2B  with at least the housing  210  and the baseplate  214  removed. In  FIG. 3 , a counterbalance  226  is positioned on a bottom surface of the proof mass  202  and also suspended above the substrate offset space. The counterbalance  226  reduces the pedulosity of the proof mass  202  and, therefore, a sensitivity of the proof mass  202  to undesired inputs, such as vibrations. In further examples, mechanical stops  302   a ,  302   b ,  302   c ,  302   d  may be coupled to the counterbalance  226  to prevent large excursions of the proof mass  202  from a predefined area of travel. That is, the mechanical stops  302   a ,  302   b ,  302   c ,  302   d  may be positioned to define a limit of travel of the proof mass  202  relative to the substrate  222  and within the detector  200 . For example,  FIG. 3  shows each of the mechanical stops  302   a ,  302   b ,  302   c ,  302   d  coupled to a side surface of the counterbalance  226 . While shown as having one of the mechanical stops  302   a ,  302   b ,  302   c ,  302   d  at each corner of the rectangular counterbalance  226 , in various other examples, the mechanical stops  302   a ,  302   b ,  302   c ,  302   d  may be positioned at other locations on the counterbalance  226 , or may be attached to the housing  210 . 
     Returning to  FIGS. 2A and 2B , the flux concentrators  208  can operate to focus the received electric field on the source of concentrated charge  204 . As shown, the flux concentrators  208  may be integrated within the housing  210 , and in particular, attached to an interior surface of the housing  210 . In other examples, the flux concentrators  208  may be attached to the substrate  222  or the baseplate  214 . In various examples, the flux concentrators  208  magnify the intensity of the electric field near the location where the electric field intercepts the source of concentrated charge  204 . The flux concentrators  208  may each be composed of metal, or a material with a high dielectric constant, which routes the flux through a spatial volume thereof. For example, each flux concentrator  208  may be composed of copper. By positioning the flux concentrators  208  near the source of concentrated charge  204 , the electric field is concentrated to provide a gain at the source of concentrated charge  204 . In the shown example, a first flux concentrator  208   a  is positioned proximate a side surface of the proof mass  202  and a second flux concentrator  208   b  is positioned proximate another, distal, side surface of the proof mass  202 . 
     In various examples, each flux concentrator  208  is positioned as close as possible to the source of concentrated charge  204  to maximize the provided gain. The performance of each flux concentrator  208  may also be enhanced by increasing a length and/or an area of the respective flux concentrator  208  to maximize the amount of flux received and directed to the source of concentrated charge  204 . Relative to the housing  210 , each flux concentrator  208  may be internal, external, or a combination of both depending upon the level of enhancement desired. In addition to the flux concentrators  208 , in certain examples the electric field detector  200  may include additional signal processing components which enhance the ability of the electric field detector  200  to resolve small signals. Such components are further described below with reference to at least  FIG. 6 . According to certain other examples, the one or more sense electrodes (or sets of one or more sense electrodes) and the one or more drive electrodes (or sets of one or more drive electrodes) that provide the capacitive readout may be replaced by other structures that are configured to measure the torque or torques on the proof mass  202  from a received electric field. For instance, the electric field detector  200  may include one or more sensors that measure the torque by its effect on a frequency of one or more of the plurality of supports  206 , or one or more sensors that optically measure a displacement of the proof mass  202 . 
     In some examples, the electric field detector  200  includes one or more sense electrodes  502  to determine a distance between the proof mass  202  and the one or more sense electrodes  502 . Furthermore, in some examples, the electric field detector  200  includes sense electrodes configured to determine torsional movement of the proof mass  202  without a distance between the proof mass  202  and the sense electrodes changing. For example, a capacitance between the proof mass  202  and the sense electrodes may change as a capacitive coupling between the proof mass  202  and the sense electrodes changes due to changes in an overlap between the proof mass  202  and the sense electrodes caused by torsional movement of the proof mass  202 . For example, the proof mass  202  and the sense electrodes may collectively include a comb-like structure having elements (for example, silicon-based elements) that slide past one another as the proof mass  202  rotates, thereby causing variations in a capacitance between the proof mass  202  and the sense electrodes sensed by the sense electrodes. Thus, the sense electrodes may sense rotation of the proof mass  202  about all three dimensions of three-dimensional space. 
     As also shown in  FIGS. 2A and 2B , in various examples the electric field detector  200  includes the housing  210 . The housing  210  is positioned to encompass the other components of the electric field detector  200 , such as the proof mass  202 , the plurality of supports  206 , the one or more flux concentrators  208 , the one or more anchors  212 , the substrate  222 , the sense electrodes, the drive electrodes, and the one or more electrical contacts  216 , among other components. In certain examples, the housing  210  may provide a vacuum environment which reduces the sensitivity of the electric field detector  200  to acoustic coupling and air damping, which reduces Brownian noise. A vacuum environment also helps to ensure that a minimal charge is maintained by preventing the dielectric breakdown of air within the electric field detector  200 . In addition to these benefits, the housing  210  protects the discussed components of the electric field detector  200  from dust, moisture, and other contaminants. In one example the housing  210  may be formed from transparent glass to permit displacement of the proof mass  202  to be measured optically. 
     According to an example, a scale factor of the electric field detector  200  may be increased by using one or more bias voltages to create an electrostatic spring with a negative stiffness relative to the mechanical stiffness of the supports  206 . A strong bias voltage on a sense electrode, drive electrode, and/or other electrodes positioned near the proof mass  202  and/or source of concentrated charge  204  generates a force (for example, negative spring force) which is opposite of the mechanical spring force of the supports  206 , and thereby decreases the overall stiffness of the MEMS structure. Accordingly, when summed, the negative stiffness reduces the total stiffness of the electric field detector  200  and increases the response of the proof mass  202  to a received electric field. Such an approach provides the benefit of increased performance without the loss of robustness, which would otherwise result if the stiffness of each of support  206  was mechanically reduced. While in certain examples the electric field detector  200  may include additional electronics to create a negative spring by force inputs (for example, a control loop or a magnetic field), application of bias voltages to create an electrostatic spring provides the benefit of low-noise performance and reduced complexity. 
     As discussed herein, multiple electric field detectors  200  may be integrated into an array to enhance electric field detection performance. That is, an array of electric field detectors may be arranged to improve the ability of each individual detector to sense weak electric field signals and/or to measure a spatial distribution of electric fields around the user or patient.  FIG. 4  shows one example of an array of electric field detectors incorporated within a headset  400 . As shown, the headset  400  may be placed over the head of a patient, or user, to detect bio-electrical signals generated by the brain. It is appreciated that other implementations may be designed to detect bio-physical signals generated by other areas of the body of a patient or user, such as the heart, nerves, or muscles, to name a few examples. 
     In the example of  FIG. 4 , each electric field detector  402  within the array is coupled to the other electric field detectors  402  such that received electric field signals are coherently amplified while noise within the array remains incoherent. However, in certain other examples each electric field detector  402  may operate independently to individually measure the amplitude and phase of the received signal. 
     Referring to  FIG. 4 , each electric field detector  402  is located between a shield layer  404  (for example, a faraday cage) and the scalp of the patient or user. Each electrical field detector  402  is closely spaced relative to the other electric field detectors  402  (for example, approximately 1 cm apart) to maximize the spatial resolution of the array. On an opposite side of the shield  404  relative to the electric field detectors  402 , additional electronics  406  can be positioned. Such an arrangement isolates the electric field detectors  402  from interfering effects which may arise from the operation of the additional electronics  406 . For example, the additional electronics may include one or more auxiliary sensors, and/or circuitry for communicating with a control circuit, as discussed below. In this way, the shield  404  isolates the electric field detectors  402  from external noise sources (for example, a 60 Hz power line noise), as well as, system components which may generate interference. 
     Each of the electric field detectors  402  and additional electronics  406  may be connected to a communication network via an electrical connection  408  that routes measured signals to a central location for processing. Auxiliary sensors may also be incorporated within the electronics  406  of the headset to measure effects which may introduce errors in the intended bio-electrical measurement (for example, one or more external parameters). For example, inertial sensors and/or temperature sensors can be co-located with the electric field detectors  402  to measure electric fields, accelerations (for example, patient movement), or temperature. Likewise, additional sensors, such as blink detectors or other physiological monitors can be incorporated within the headset  400  to improve the accuracy and performance of the array. As shown, components of the headset  400  are embedded within a cap  410  which provides structure and supports the various components. The cap  410  may include padding and other helmet features (for example aesthetically pleasing covers) to increase comfort and improve the user experience. 
     Accordingly, the array of electric field detectors may provide numerous benefits in various applications. For instance, the array may provide diagnostic information for educational applications, training applications, and cognitive enhancement applications. Moreover, current diagnostic techniques and approaches for neurological conditions may be enhanced by the information ascertained by the array of electric field detectors  402 . For instance, the array of electric field detectors  402  enhances current techniques for treating ADHD, autism, dyslexia, depression, insomnia, impulsivity, and anxiety. Other relevant clinical applications include, but are not limited to, pain management, mental health treatment, epilepsy, and dementia, among other brain disorders. In other examples, electric field detectors may be implemented in other applications, including muscle monitoring. For example, electric field detectors may be implemented to monitor electric fields generated by skeletal muscles, such as the calves, quadriceps, and so forth, or other muscles, such as the heart. 
       FIG. 12  illustrates a block diagram of a sensing system  1200  according to an example. The sensing system  1200  may be particularly well-suited to measure biological electric fields, such as those generated by muscles of a patient. The sensing system  1200  includes one or more electric field detectors  1202  (which may include, or be implemented substantially similarly to, the electric field detector  200 ), one or more auxiliary sensors  1204 , a controller  1206 , a communication interface  1208 , a power source  1210 , power conditioning circuitry  1212 , and a substrate  1214 . 
     The one or more electric field detectors  1202  are configured to detect parameters of an electric field, such as a direction, phase, and/or magnitude. For example, the one or more electric field detectors  1202  may include electric field detectors substantially similar to the electric field detector  200  and/or the electric field detector  1100 . In some examples, the one or more electric field detectors  1202  may include single-axis electric field detectors, as discussed above with respect to the electric field detector  200 , and/or may include multi-axis (for example, two- and/or three-axis) electric field detectors configured to detect parameters of an electric field in multiple axes, as discussed in greater detail below. Orientations of the one or more electric field detectors  1202  may be selected to acquire electric field information in a desired number and combination of dimensions. For example, the one or more electric field detectors  1202  may include three orthogonally oriented single-axis electric field detectors to acquire electric field information in all three dimensions of three-dimensional space. In another example, the one or more electric field detectors  1202  may include a single three-axis electric field detector to acquire electric field information in all three dimensions of three-dimensional space. In another example, the one or more electric field detectors  1202  may include two two-axis electric field detectors to acquire electric field information in all three dimensions of three-dimensional space, with redundancy in one dimension. In other examples, the one or more electric field detectors  1202  may include any number of electric field detectors in any combination of orientations. Furthermore, the one or more electric field detectors  1202  may be positioned in various permutations. For example, multiple electric field detectors may be co-located or placed in spatial patterns to improve accuracy and sensitivity by averaging measurements, or by performing inverse modeling to determine spatiotemporal properties of biological signal sources. 
     The one or more auxiliary sensors  1204  are configured to sense auxiliary information. Similar to the auxiliary sensors  230 , the one or more auxiliary sensors  1204  may aid in compensating for external accelerations, and/or effects from other external parameters, by directly measuring the external parameter(s), and adjusting information indicative of a measured electric field to compensate for the external parameter(s). For instance, in addition to external accelerations and/or movements, the one or more auxiliary sensors  1204  may measure at least one of noise, ambient temperature, or vibrations. Accordingly, the one or more auxiliary sensors  1204  may include an accelerometer, gyroscope, magnetometer, temperature sensor, noise sensor, optical sensor, or other sensor, to name a few examples. The controller  1206  may receive measurements from the one or more auxiliary sensors  1204  and use one or more of various filtering techniques (for example, digital signal processing filter techniques), for example, to adjust the characteristic of the electric field sensed by the one or more electric field detectors  1202  to compensate for the effect(s) of the measured external parameter(s) on the measured characteristic of the electric field. In various examples, adjusting the measured characteristic of the electric field may include applying a filter to remove the effect of the external parameter(s). For example, movement of the sensing system  1200  may cause certain undesirable motion artifacts to appear. By identifying movement of the sensing system  1200  with the one or more auxiliary sensors  1204 , these motion artifacts may be identified and eliminated as having been caused by movement of the sensing system  1200 . The particular arrangement and position of auxiliary sensors within the sensing system  1200  may vary based on the particular external parameter desired to be measured, as well as the particular architecture of the sensing system  1200  itself. 
     The controller  1206  includes control circuitry to control operation of the sensing system  1200 . The controller  1206  may include, or be an example of, a control circuit as discussed herein, and as discussed below with respect to  FIG. 6 . The controller  1206  is configured to determine, based on information received from the one or more electric field detectors  1202  and/or the one or more auxiliary sensors  1204 , characteristics of the electric field as discussed herein. In one example, each of the one or more electric field detectors  1202  includes a control circuit communicatively coupled to the controller  1206  to send information indicative of an electric field. The controller  1206 , in turn, may determine characteristics of the electric field based on the received information. The controller  1206  may also be configured to control certain aspects of the one or more electric field detectors  1202 . For example, the one or more electric field detectors  1202  may include one or more electric field detectors having a dynamic electric dipole that is selectively polarized at a frequency controlled by the controller  1206  in combination with control circuitry of each respective one of the one or more electric field detectors  1202 . 
     Using data stored in associated memory, the controller  1206  also executes one or more instructions stored on one or more non-transitory computer-readable media that may result in manipulated data. In some examples, the controller  1206  may include one or more processors, field-programmable gate arrays, or other types of controllers. In one example, the controller  1206  is or includes a commercially available, general-purpose processor. In another example, the controller  1206  performs at least a portion of the operations discussed above using an application-specific integrated circuit tailored to perform particular operations in addition to, or in lieu of, a general-purpose processor. As illustrated by these examples, examples in accordance with the present invention may perform the operations described herein using many specific combinations of hardware and software and the invention is not limited to any particular combination of hardware and software components. 
     The communication interface  1208  is configured to enable communication with one or more external entities. For example, the communication interface  1208  may include an antenna configured to output electromagnetic radiation (for example, radio waves) encoding certain information to an external entity, such as a user device. The controller  1206  may control the communication interface  1208  to output electromagnetic radiation encoding information indicative of parameters of an electric field. For example, the controller  1206  may control the communication interface  1208  to output electromagnetic radiation encoding a direction, magnitude, and/or phase of an electric field produced by a patient&#39;s muscles, such as the patient&#39;s heart. 
     The power source  1210  is configured to provide electrical power to components of the sensing system  1200 . For example, the power source  1210  may include one or more batteries, which may be rechargeable via a wired or wireless medium. 
     The power conditioning circuitry  1212  is configured to condition power provided by the power source  1210 . Conditioning the power provided by the power source  1210  may include rectifying, inverting, and/or converting power provided by the power source  1210 . For example, where the one or more electric field detectors  1202  include a dynamic electric dipole such as the electric field detector  1100 , the power conditioning circuitry  1212  and/or the controller  1206  may invert DC power received from the power source  1210  to provide AC power at a desirable frequency to the electrodes  1106 . In another example, the power conditioning circuitry  1212  may include one or more power converters configured to step a voltage up or down to a desired level. 
     The substrate  1214  is configured to couple the sensing system  1200  to a patient. For example, the substrate  1214  may include an adhesive patch having an adhesive side to removably adhere to a patient&#39;s body, such as on a patient&#39;s chest, legs, arms, and so forth. In another example, the substrate  1214  may include a patient&#39;s clothing, including athletic wear (for example, an athlete&#39;s padded uniform) and casual wear (for example, a shirt, pants, and so forth). In another example, the substrate  1214  may include a compressive material, such as a band, watch, or strap, to compress around a portion of a patient&#39;s body. In other examples, the substrate  1214  may include any other substrate to facilitate coupling of the sensing system  1200  to a patient&#39;s body. 
     The substrate  1214  may fully or partially encapsulate or otherwise include the components  1202 - 1212 . In some examples, the substrate  1214  may be at least partially removable from other components  1202 - 1212  of the sensing system  1200 . For example, the components  1202 - 1212  may be removably coupled to the substrate  1214  via a removable coupling mechanism such as a snap, a clip, an adhesive, hook-and-loop fastener, a zipper, and so forth. In these examples, the components  1202 - 1212  may be encapsulated, housed, or otherwise included within another substrate or encapsulate that is configured to be removably coupled to the substrate  1214 . Removably coupling the substrate  1214  to the components  1202 - 1212  may be beneficial where, for example, the substrate  1214  directly contacts a patient&#39;s body. It may be undesirable for the substrate  1214  to subsequently directly contact another patient&#39;s body, but the components  1202 - 1212  may still be operational. Thus, the substrate  1214  can be removed and disposed of, and the components  1202 - 1212  can be coupled to another substrate, substantially similar to the substrate  1214  but not having been previously used with a patient, which may be subsequently coupled to another patient to reduce waste of the components  1202 - 1212 . 
     In some examples, the sensing system  1200  may be externally coupled to a patient&#39;s body. In other examples, the sensing system  1200  may be configured to be inserted into a patient&#39;s body. For example, the sensing system  1200  may be, or be included within, an implantable device. In these examples, the substrate  1214  may encapsulate the components  1202 - 1212  of the sensing system  1200 , and may be formed of a biocompatible material or materials that do not adversely affect a patient&#39;s body. In another example, the sensing system  1200  may be, or be included within, a catheter (which may be included within an “implantable device”), or other device that is temporarily or removably inserted into a patient&#39;s body. In these examples, the substrate  1214  may similarly be formed of a biocompatible material or materials that do not adversely affect a patient&#39;s health. 
     In various examples, the substrate  1214  may include, or be coupled to, a shielding component configured to shield the one or more electric field detectors  1202  from external signals. For example, the substrate  1214  may include, or be coupled to, a metal shielding layer to encapsulate at least a portion of the sensing system  1200  to attenuate or block external electrical fields not generated by the patient from reaching the sensing system  1200 . In another example, the substrate  1214  may include a waterproofing material, or may be coupled to a waterproof encapsulate, to prevent moisture from adversely affecting components of the sensing system  1200 . 
     Accordingly, in various examples, components  1202 - 1212  of the sensing system  1200  may be coupled, contained, or included, removably or non-removably, to or within the substrate  1214 . The substrate  1214 , in turn, may be coupled to a patient. In other examples, components of a sensing system may be distributed rather than being coupled, contained, or included in a single substrate. 
       FIG. 13  illustrates a distributed sensing system  1300  according to another example. The sensing system  1300  may be particularly well-suited to measure internal biological electric fields, such as those generated by a patient. The sensing system  1300  includes an implantable portion  1302 , which is configured to be inserted into a patient&#39;s body, and an external portion  1304 , which is configured to be external to a patient&#39;s body. The implantable portion  1302  is communicatively and/or electrically coupled to the external portion  1304  via a connection  1303 , which may include wired and/or wireless media. The implantable portion  1302  includes one or more electric field detectors  1306  (which may include, or be implemented substantially similarly as, the electric field detector  200  and/or  1100 ), and optionally includes one or more first optional auxiliary sensors  1308 , a first optional controller  1310 , a first optional communication interface  1312 , a first optional power source  1314 , first optional power conditioning circuitry  1316 , and a first housing  1318 . The external portion  1304  optionally includes one or more second optional auxiliary sensors  1320 , a second optional controller  1322 , a second optional communication interface  1324 , a second optional power source  1326 , second optional power conditioning circuitry  1328 , and a second housing  1330 . 
     Components  1308 - 1316  and  1320 - 1328  are described as optional components to indicate that the indicated components may be included in either (or both) of the implantable portion  1302  or the external portion  1304 . Power and/or information may be exchanged via the connection  1303  depending on which components are included within which of the portions  1302 ,  1304 . For example, where the first optional power source  1314  is included in the implantable portion  1302  and includes an energy source, such as a battery, the first optional power source  1314  may provide power to components of the internal portion  1302 . Furthermore, electrical power may be sent from the external portion  1304  via the connection  1303  to charge the first optional power source  1314  and/or provide auxiliary power to other components of the implantable portion  1302  in addition to power provided by the first optional power source  1314 . The electrical power may be sent by the second optional power source  1326 , which may be included in the external portion  1304  and may include a power source such as a battery, mains utility power, or another power source. Alternatively, the first optional power source  1314  may include a non-rechargeable energy storage device, and the second optional power source  1326  may be omitted completely, or may be included to provide auxiliary power to other components of the implantable portion  1302  in addition to power provided by the first optional power source  1314 . In another example, the first optional power source  1314  may not be included in the internal portion  1302 , and the second optional power source  1326  may be included in the external portion  1304  to provide electrical power to components of the internal portion  1302  (including, for example, the one or more electric field detectors  1306 ) via the connection  1303 . For example, the second optional power source  1326  may be an external power source, such as an energy storage device (for example, a battery), mains utility power, or another power source. 
     In another illustrative example, the first optional controller  1310  may be omitted from the implantable portion  1302 , the first optional communication interface  1312  may be included in the implantable portion  1302 , and the second optional controller  1322  and the second optional communication interface  1324  may be included in the external portion  1304 . In this example, information acquired by components of the implantable portion  1302  (for example, the one or more electric field detectors  1306  and/or the first optional auxiliary sensors  1308 , if included) may be communicated, from the first optional communication interface  1312  to the second optional communication interface  1324  via the connection  1303 , to the second optional controller  1322 . For example, the one or more electric field detectors  1306  and/or the first optional auxiliary sensors  1308  may communicate information indicative of electric field information (for example, capacitance information determined by the one or more electric field detectors  1306  and/or movement information, such as acceleration information, determined by the first optional auxiliary sensors  1308 ) to the second optional controller  1322 . The second optional controller  1322  may, in turn, determine electric field information based on the received information. In other examples, the first optional controller  1310  may be included in the implantable portion  1302 , and the first optional controller  1310  may determine electrical field information and communicate the electrical field information and/or other information indicative of the electrical field information to the second optional controller  1322 , or another entity, via the connection  1303 . 
     Similar principles apply to other optional components of the sensing system  1300 , that is, either or both of the implantable portion  1302  and the external portion  1304  may include the optional components depending on an implementation of the sensing system  1300 . In various examples, components of the sensing system  1300  may include additional components not specifically identified. For example, where the sensing system  1300  is integrated with a medical device, such as a catheter, the implantable portion  1302  and the connection  1303  may include additional components to enable the traditional functions of the catheter. Furthermore, components of the sensing system  1300  may be adapted for the traditional functions of a medical device in which the sensing system  1300  is integrated. For example, the first housing  1318  and/or the connection  1303  may include a biocompatible material or materials if the first housing  1318  and/or the connection  1303  are to be inserted into a patient&#39;s body. 
     Furthermore, it is to be appreciated that the connection  1303  includes wireless media in some examples. For example, the internal portion  1302  may be an implantable device configured to receive power and/or exchange information with the external portion  1304  via the connection  1303  in a wireless format. For example, the external portion  1304  may provide wireless power to the internal portion  1302  via the connection  1303 , and the external portion  1304  may receive information (for example, information indicative of electric field information) via a wireless medium, such as electromagnetic radiation (for example, via radio waves). 
     Referring now to  FIG. 5 , illustrated is a plan view of one example of sense electrodes  502   a ,  502   b  (collectively “sense electrodes  502 ”) and drive electrodes  504   a ,  504   b  (collectively “drive electrodes  504 ”) of the electric field detector  200  (which, as discussed above, may be implemented in connection with the electric field detector  1100 , the one or more electric field detectors  1202 , and/or the one or more electric field detectors  1306 ) illustrated in  FIGS. 2A and 2B . For simplicity,  FIG. 5  illustrates the sense electrodes  502  and the drive electrodes  504  implemented in an example of the electric field detector  200  in which the electric field detector  200  detects aspects of an electric field in one dimension only, that is, in which the proof mass  202  only rotates about a single torque axis. In other examples, in which the electric field detector  200  is configured to detect aspects of an electric field in multiple orthogonal dimensions, the sense electrodes  502  and drive electrodes  504  may include additional electrodes, substantially similar to the electrodes  502 ,  504 , oriented in an orthogonal dimension from the electrodes  502 ,  504 . For example, whereas the sense electrodes  502   a ,  502   b  are positioned along an x-axis, an additional set of sense electrodes could be implemented and positioned along the y-axis to detect an orthogonal component of an electric field. The additional set of sense electrodes could be implemented in the same plane as the sense electrodes  502 , or implemented in a different plane as the sense electrodes  502  (for example, in a different plane along the z-axis). 
     Returning to the example illustrated by  FIG. 5 ,  FIG. 5  illustrates the electrical connections between the sense electrodes  502  and the corresponding electrical contacts  216 , and the electrical connections between the drive electrodes  504  and the corresponding electrical contacts  216 . As previously discussed, leads  218  may couple electrical contacts  216  on the substrate  122  and electrical contacts  216  on the baseplate  214  to the control circuit. For the convenience of illustration, leads  218  are not shown in  FIG. 5 . As discussed above with reference to  FIGS. 2A and 2B , in various examples the sense electrodes  502  and the drive electrodes  504  are formed on the substrate  222 , and in particular, within the substrate offset space beneath the proof mass  202 .  FIG. 5  is described with continuing reference to the electric field detector  200  illustrated in  FIGS. 2A and 2B , and the components thereof. 
       FIG. 5  illustrates a first sense electrode  502   a  (for example, a left sense electrode), a second sense electrode  502   b  (for example, a right sense electrode), a first drive electrode  504   a  (for example, a left torquer), and a second drive electrode  504   b  (for example, a right torquer). As further discussed with reference to  FIG. 7A-7C  and  FIGS. 8A-8C , each of the first sense electrode  502   a , second sense electrode  502   b , first drive electrode  504   a , second drive electrode  504   b , and electrical contacts  216  may be applied as a metallization layer to the substrate  222 . For instance, each sense electrode  502 , each drive electrode  502 , and/or each electrical contact  216  may be a layer of chrome, platinum, or gold on the substrate  222 . As previously described, one or both of the sense electrodes  502  may be used to measure a change in capacitance (for example, electrical capacitance) relative to the proof mass  202  as a result of torsional movement of the proof mass  202 . One or both of the drive electrodes  504  may be used to produce a feedback torque on the proof mass  202  and reposition the proof mass  202 . 
     In one example, the two sense electrodes  502   a ,  502   b  are used for a differential capacitance measurement, and the two drive electrodes  504   a ,  504   b  are used as torquers for force feedback during closed loop operation. Each sense electrode  502  and drive electrode  504  is interposed between a pair of respective electrical contacts  216  and extended along a length of the substrate  222 . While shown in  FIG. 5  as a pair of sense electrode plates and a pair of drive electrode plates, each plate having a substantially rectangular shape, in various other examples any suitable number of sense electrodes  502  and drive electrode  504  may be used (for example, by increasing a number of sense electrodes to detect aspects of an electric field in multiple dimensions), and each of the sense electrodes  502  or drive electrodes  504  may have any suitable shape. Moreover, in certain examples the first sense electrode  502   a  and the first drive electrode  504   a  may be connected and act as a single large electrode to maximize performance when not operating in a closed loop mode of operation. In such an example, the second sense electrode  502   b  and the second drive electrode  504   b  may be coupled in a similar manner. In certain examples, the sense electrodes  502  and the drive electrode  504  may be reversed and their relative areas chosen to optimize the relative level of performance between the drive and sense operations. In one example, the sense electrodes  502   a ,  502   b  (for example, the outer-positioned electrodes) act on the plurality of supports  206  of the detector  200 , and therefore may have a greater effectiveness. 
     In various examples, each sense electrode  502  and drive electrode  504  may include a respective guard ring  506 . As shown in  FIG. 5 , the proof mass  202  may also have a guard ring  508 . Each guard ring  506  substantially surrounds the respective sense electrode or drive electrode and separates that sense electrode or drive electrode from the other sense electrode and drive electrode. In one example, each the guard ring  506  is a thin metal track that traces the perimeter of the corresponding plate or electrode. Each guard ring  506 ,  508  substantially eliminates direct-current (DC) current and low-frequency leakage currents from unintentionally affecting the corresponding sense electrodes  502 , drive electrodes  504 , or proof mass  202 . DC current and low-frequency leakage current may limit the dynamic range of the electric field detector  200  and may create low-frequency noise by producing undesired voltages in the source impedances.  FIG. 5  further shows a ground contact  510  for the proof mass  202 . 
     Turning now to  FIG. 6 , shown is one example of a control circuit  600  that may be coupled to the electric field detector  200  illustrated in  FIGS. 2A and 2B , or that may be included in or be an example of the controllers discussed herein, including the controllers  1206 ,  1310 , and  1322 , to detect the characteristics of an electric field received at the detectors  200 ,  1100 ,  1202 ,  1306 , and/or provide one or more control signals (for example, for driving the drive electrodes). For instance, the control circuit may be coupled to the contacts  216  illustrated in  FIGS. 2A and 2B .  FIG. 6  is discussed with continuing reference to the electric field detector  200  of  FIGS. 2A and 2B , and the components thereof, for purposes of explanation. 
     In certain examples, the control circuit  600  may include any processor, multiprocessor, or controller. Furthermore, in some examples, the control circuit  600  may be coupled to an external controller, such as the controllers  1206 ,  1310 ,  1322 . The processor may be connected to a memory and a data storage element. The memory stores a sequence of instructions coded to be executable by the processor to perform or instruct the various components discussed herein to perform the various processes and acts described herein. For instance, the control circuit  600  may communicate with, and provide one or more control signals to the sense electrodes and the drive electrodes of the electric filed detector via the contacts  216  and the leads  218 . The memory may be a relatively high performance, volatile random-access memory such as a dynamic random-access memory or static random-access memory. However, the memory may include any device for storing data, such as a disk drive or other nonvolatile storage device. 
     The instructions stored on the data storage may include executable programs or other code that can be executed by the processor. The instructions may be persistently stored as encoded signals, and the instructions may cause the processor to perform the functions and processes described herein, such as providing one or more control signals to generate a feedback torque. The data storage may include information that is recorded, on or in, the medium, and this information may be processed by the processor during execution of instructions. The data storage includes a computer readable and writeable nonvolatile data storage medium configured to store non-transitory instructions and data. In addition, the data storage includes processor memory that stores data during operation of the processor. 
     In the illustrated example, the control circuit  600  includes a precision square-wave generator  602  which is coupled to a first filter  604 . The precision square-wave generator  602  generates a signal which is converted to a sine wave by the first filter  604 . The first filter  604  may include any suitable filter designed to accept a square-wave input and provide a sinusoidal output. For instance, one example is a low-Q active bandpass filter with a notch filter to reduce the third-order harmonic. In various examples, the first filter  604  has a very low amplitude sensitivity to temperature, such as 1-3 ppm per degree Celsius. The first filter  604  is coupled to an inverting amplifier  606  which has an adjustable gain and a nominal gain of −1. Accordingly, an output of the first filter  604  and the inverting amplifier  606  form a low-noise differential sine-wave carrier generator. 
     As shown in  FIG. 6 , the carrier generator may be coupled to each of the sense electrodes (for example, shown as readout capacitors  608   a ,  608   b , collectively “readout capacitors  608 ”) to excite the readout capacitors  608  in order to up-convert (for example, increase a frequency) an electronics signal produced by the received electric field. In various examples, by up-converting the received electric field information, the information is converted to a frequency where amplifier noise is significantly lower. Moreover, the up-conversion reduces the sensitivity of the electric field to current noise sources in a preamplifier  610  coupled to the readout capacitors  608 . While not illustrated in  FIG. 6 , in many instances the control circuit  600  may include one or more passive high-pass filters interposed between the outputs of the carrier generator and the readout capacitors  608  to reduce low-frequency voltage noise coupled to the readout capacitors  608  from the carrier generator. Such an arrangement offers the benefit of reduced low-frequency torque noise. 
     Referring to the electric field detector  200  of  FIG. 2A , in the absence of an electric field, there will be no torque on the proof mass  202  (in an ideal case). In such a situation, no electric field information is passed from the readout capacitors  608  (sense electrodes  502  in  FIG. 5 ) to the preamplifier  610 . However, when an electric field is present, the readout capacitors  608  provide a measured signal to the preamplifier  610 , which in turn provides an output of a carrier signal amplitude-modulated by the electric field (for example, a double-sideband suppressed carrier signal). 
     In various examples, the control circuit  600  includes a second amplifier  612  and a second filter  614  coupled to the output of the preamplifier  610 . For instance, the second amplifier  612  may include a low-noise instrumentation amplifier with an input-referred noise density that is substantially less than the output-referred noise density. For example, the second amplifier  612  may include, or be coupled to, a chopping amplifier configured to reduce instrumentation noise. The carrier signal amplitude-modulated by the electric field is received and amplified by the second amplifier  612  before being filtered by the second filter  614  and received at a demodulator  618 . According to certain examples, the second filter  614  includes a band-pass filter which has a low quality factor to reduce the noise within the amplitude-modulated carrier signal at the third order and higher order harmonics. Accordingly, the second filter  614  provides filtering functionality to prevent higher order harmonics from affecting the noise performance of the control circuit  600  after the carrier signal has been demodulated. In certain implementations, the control circuit  600  may also include a third amplifier  616  which is coupled to an output of the second filter  614  and configured to add an additional gain to the carrier signal amplitude-modulated by the electric field information. While illustrated in  FIG. 6  as separated from the second filter  614 , in certain examples the third amplifier  616  provides additional AC gain and may be incorporated into the second filter  614 . 
     As shown in  FIG. 6 , the control circuit  600  includes a demodulator  618  and comparator  620  which are coupled to form a switching (or square wave) demodulator. In  FIG. 6 , the switching demodulator is coupled to an output of the third amplifier  616 . The demodulator  618  drives a controller  622 , which is coupled to the output of the demodulator  618 . In some examples, the controller  622  may include an Integral-Derivative (ID) controller, a Proportional-Integral-Derivative (PID) controller, or any other suitable predictive controller. In one example, the controller  622  drives a torque generator  624  which produces a bias voltage at each respective torque generator electrode (for example, drive electrodes  504   a ,  504   b  illustrated in  FIG. 5 ). In particular, the torque generator may produce respective torque generator voltages of (BIAS+K*−V C ) and (BIAS−K*V C ), where “BIAS” is a bias voltage, “K” is a scaling constant, and “V C ” is the output of the controller  622 . For example, the torque generator  624  may produce a substantially constant bias voltage having a nominal value near one-half of the positive or negative supply voltage. While in the illustrated example, the torque generator  624  includes summation blocks  634 ,  638 , an inverting gain  636 , and an adjustable gain  632  for the purpose of illustration, in various other examples the torque generator  624  may be implemented with various other suitable components. 
     Accordingly, the applied torque, which is proportional to the square of the voltage, is directly proportional to the output of the controller  622 . Such a biasing arrangement achieves a linearization of the closed-loop feedback torque applied to the proof mass  202  with respect to the output of the controller  622 . This arrangement results in a linear control loop and permits a linear readout of the electric field information. In certain examples, the control circuit  600  may further include one or more passive low-pass filters (not shown) interposed between the torque generator  624  and the torque generator electrodes in order to reduce carrier-band noise applied to the torque generator electrodes. 
     As further illustrated in  FIG. 6 , the control circuit  600  may include a baseband filter  626  coupled to the output of the controller  622 . For example, the baseband filter  626  may include a bandpass filter having a passband selected to extract the electric field information within the desired bandwidth from the output of the demodulator  618 . The output of the baseband filter  626  may then be amplified by a fourth amplifier  628  and provided to an output of the control circuit  600  or one or more downstream diagnostic electronics. In at least one example, the fourth amplifier  628  is designed such that most of a variable voltage range of the amplifier  628  corresponds to a maximum expected in-band field strength of the electric field. Such a design provides the benefit of reduced noise. For instance, the fourth amplifier  628  may include a high-gain amplifier that has a gain of about 100. The parameters of the fourth amplifier  628  may be selected in conjunction with the parameters of the baseband filter  626  to select and amplify a desired frequency band (for example, a frequency band associated with brain activity (0.5 Hz-100 Hz)). As shown, in certain examples the control circuit  600  may also include a fifth amplifier  630  to provide an unfiltered output for diagnostic purposes. 
     Though the features within  FIG. 6  are illustrated as blocks within a block diagram, unless otherwise indicated, the features may be implemented as signal processing circuitry, and may be implemented with one or more specialized hardware components or one or more specialized software components. For instance, the control circuit  600  may be implemented as one of, or a combination of, analog circuitry or digital circuitry. The control circuit  600  may be composed of an array of logic blocks arranged to perform one or more of the corresponding signal processing operations described herein. In particular, the processing circuitry may be implemented by an array of transistors arranged in an integrated circuit that provides a performance and power consumption similar to an ASIC (application-specific integrated circuit) or an FPGA (field-programmable gate array). In other examples, components of the control circuit  600  may be implemented as one or more microprocessors executing software instructions (for example, predefined routines). In particular, the software instructions may include digital signal processing (DSP) instructions. Unless otherwise indicated, signal lines may be implemented as discrete analog or digital signal lines, or as a single discrete digital signal line with appropriate signal processing to process separate signals. 
     Turning now to  FIGS. 7A-7C  and  FIGS. 8A-C , illustrated is an example of a process  700  for fabricating an electric field detector, such as an example of the electric field detector  200  illustrated in  FIGS. 2A-2B  and  FIG. 3 . More particularly,  FIGS. 7A-7C and 8A-8C  illustrate a process  700  for fabricating an example of the electric field detector  200  being configured to detect aspects of an electric field in one dimension. An alternate process may apply to fabricating an example of the electric field detector  200  being configured to detect aspects of an electric field in multiple dimensions, such as by including additional acts involving the fabrication of sense electrodes. 
       FIGS. 7A-7C  illustrates the process flow and  FIGS. 8A-8C  show a state of an electric field detector during each act of the process  700 . Each act of the process  700  of  FIG. 7A-7C  is illustrated immediately adjacent the corresponding state of production of the electric field detector. Accordingly, in some examples, the electric field detector shown in  FIGS. 8A-8C  may be one implementation of the electric field detector  200  described with reference to at least  FIGS. 2A and 2B . That is, at least the source of concentrated charge, the substrate, the support(s), the proof mass, the sense electrode(s), and the drive electrode(s) described with reference to  FIGS. 8A-8C  may correspond to examples of the source of concentrated charge, the support(s), the proof mass, the sense electrode(s), and the drive electrode(s) previously described with reference to at least  FIGS. 2A and 2B , as well as, the sense electrode(s) and the drive electrode(s) described with reference to  FIG. 5 . 
     The process  700  begins at act  702  which may include the act of providing a substrate wafer  802  (referred to generally as the “substrate  802 ”). In various examples, the substrate  802  is a glass wafer. The glass wafer may be doped such that it conducts electricity at elevated temperatures (for example, about 350 degrees Celsius). The glass wafer may be composed of borosilicate, for example. In act  704 , the process  700  includes defining a well  804  (for example, a substrate offset space) in the substrate  802 . In certain examples, the substrate offset space is formed by etching the substrate  802 ; however, other processing techniques may be used, such as milling, grinding, or one or more deposition processes. For instance, the etching process may be implemented using the MESA™ etch system offered by APPLIED MATERIALS™ of Santa Clara, Calif. Areas of the substrate  802  which are not etched during act  704  may be later coupled to a flexure layer  814  or a handle layer  816  of a structure wafer  812 , as discussed below. 
     In act  706 , the process  700  may include depositing a conducting material, such as metal, on the substrate  802  to form one or more sense electrodes  806 , one or more drive electrodes  808 , and/or one or more guard rings and electrical contacts (not shown). In the shown example, the conducting material is primarily deposited within the substrate offset space  804 . For instance, each sense electrode  806  and each drive electrode  808  may be formed on a surface of the substrate  802  within the substrate offset space  804 . As discussed with reference to  FIGS. 2A and 2B , each sense electrode  806  may be configured to measure a change in capacitance within the substrate offset space  804  (for example, between the sense electrode and a proof mass), and each drive electrode  808  may be configured to act as a closed loop torquer on the proof mass. Each guard ring is formed on the substrate  802  to substantially surround a corresponding one of the sense electrodes  806  or drive electrodes  808  and isolate that respective sense or drive electrode plate  806 ,  808  from the effects of DC current and low-frequency leakage currents. In other examples of a fabrication process, depositing a conducting material to form one or more sense electrodes may include forming an a different number of sense electrodes (for example, additional sense electrodes) where an electric field detector being fabricated is configured to detect aspects of an electric field in multiple dimensions. 
     In act  708 , the process  700  may include conditioning the surface(s) of one or more sense electrodes  806  and/or drive electrodes  808  to increase the surface texture thereof. In one example, act  708  may include applying one or more small metal bumps  810  to the surface of the sense electrodes  806  and/or drive electrodes  808 . The increase in surface texture decreases the holding force between the substrate  802  and the structure wafer  812  by reducing the contact area between the substrate  802  and the structure wafer  812 . 
     In act  710 , the process  700  may include providing a structure wafer  812 , such as an SOI wafer. While an SOI wafer is used as one example for the purpose of explanation, in various other examples other suitable structure wafer materials may be used, such as quartz, polysilicon, etc. In the shown example of  FIGS. 8A-8C , the structure wafer  812  includes a flexure layer  814  and a handle layer  816  separated by a buried oxide layer  818 . In one example, the flexure layer  814  is about 400 μm thick (for example, ±2 μm thickness), the handle layer  816  is about 300 μm thick (for example, ±2 μm thickness), and the buried oxide  818  is about 2 μm thick (for example, ±1 μm thickness). 
     Referring to  FIG. 7B  and  FIG. 8B , in act  712  the process  700  may include defining a proof mass  820 , a plurality of supports  822 , and/or one or more anchors  824  in the structure wafer  812 . In the shown example of  FIG. 8B , each support  822  is interposed between the proof mass  820  and a respective anchor  824 . In certain examples, the proof mass  820 , the plurality of supports  822 , and/or one or more anchors  824  are formed by etching the flexure layer  814  of the structure wafer  812 ; however, other processing techniques may be used, such as milling, grinding, or one or more deposition processes. In certain examples, a Deep Reactive Ion Etch (DRIE) process may be used with a dry etch tool and Inductively Coupled Plasma (ICP) to define each of the proof mass  820 , supports  822 , and the anchors  824 . In one example, the ICP etch may also define one or more holes in the flexure layer  814 . Each hole may be used to electrically connect the flexure layer  814  and the handle layer  816 , as described during later processing acts of  FIG. 7A-7C . In  FIG. 7B , the flexure layer  814  is shown as having a hole  832   a  within the proof mass  820  and a hole  832   b ,  832   c  within each anchor  824 . 
     In act  714 , the process  700  may include selectively removing a first portion of the oxide layer  818  from the structure wafer  812 . In particular, the first portion may include those areas of the oxide layer  818  that were exposed during the etching process of act  712 . That is, in one example act  714  may include removing the exposed oxide from the holes  832   a ,  832   b ,  832   c  in the flexure layer  814 . For instance, an oxide ICP etch may be used to remove the exposed oxide. Following act  714 , in act  716  the process  700  may include defining one or more counterbalances in the handle layer  816  of the structure wafer  812 . For instance, act  716  may include etching the handle layer  816  to define a counterbalance  826  for the proof mass  820 . In act  716 , the process  700  may further include defining one or more anchor grounds  834 . Each anchor ground  834  couples a respective anchor  824  to the substrate  802 , as further discussed below with reference to act  722 . 
     In act  718 , the process  700  may include selectively metallizing each recess formed in the flexure layer  814  of the structure wafer  812  to plate the one or more formed recesses. The deposited metal  828  forms an electrical connection between the flexure layer  814  and the handle layer  816 . Following act  718 , in act  720  the process  700  includes the act of etching a second portion of the oxide layer  818 . As shown in  FIG. 8B , the second portion of the oxide layer  818  may include those sections of the oxide layer  818  that are attached to the supports  822 . Accordingly, act  720  may include releasing the supports  822  from the oxide layer  818  to suspend the proof mass  820 . In at least one example, the supports  822  are released by removing the second portion of the oxide layer  818  using a hydrofluoric acid etching process. 
     Once each of the supports  822  has been released, the process  700  may include coupling the structure wafer  812  to the substrate  802 , as shown in  FIG. 8C . In one example, the handle wafer  816  may be anodically bonded to the substrate  802 . Once the structure wafer  812  has been coupled to the substrate  802 , the proof mass  820  may be suspended above and partially within the substrate offset space  804  by the plurality of supports  822 . The anchor grounds  834  may couple the flexure layer  814  to the substrate  802  at each end of the flexure layer  814  (for example, at each anchor  824 ), where the substrate offset space  804  is substantially in the center of the substrate  802 . In an example where multiple electric field detectors are fabricated from the same of substrate  802  material and structure wafer  812  (for example, SOI wafer), the process  700  may then include dicing each sheet to separate each of the separate electric field detectors. The process  700  ends in act  724 , in which a source of concentrated charge  830  is coupled to the structure wafer  812 , and in particular, coupled to the proof mass  820 . As shown, the source of concentrated charge  830  is positioned at about the center of the flexure layer  814  such that each of the supports  822  suspends the source of concentrated charge  830  above the substrate offset space  804 . As discussed above, the source of concentrated charge  830  may be polarized before or after it has been coupled to the flexure layer  814 . Processes and acts for operating the electric field detector once it has been fabricated are discussed above with reference to the electric field detector  200  shown in  FIGS. 2A, 2B, and 3 . 
     As discussed above, in various examples the assembled electric field detector may be packed with a housing, a baseplate, and one or more electrical connections, such as the housing  210  and the baseplate  214  illustrated in  FIGS. 2A and 2B  and the electrical connections illustrated in  FIG. 5 . In various examples, the source of concentrated charge  830  may be coupled to the flexure layer  814  early in the packaging process (for example, before the sense electrodes  806  and/or drive electrodes  808  are electrically bonded to the substrate  802 ). However, in other examples, the source of concentrated charge  830  may be coupled to the flexure layer  814  as part of a vacuum sealing process with the housing, after integration in a sensor array, or during operation. In one particular example, an uncharged electret is attached to the flexure layer  814  and subsequently charged as part of a vacuum sealing process. For instance, once the detector is placed in the vacuum, an electron beam source may embed a charge on one or more surfaces of the uncharged electret to generate an electric dipole. The housing may then be attached to the baseplate of the detector to form a hermetic seal. Such a process provides the benefit of reducing air damping during operation of the detector. In other examples, charge can also be added after the housing is attached to form a hermetic seal, or continuously during operation, as is the case of an active system, where a voltage excitation is used to form an AC electric dipole on the proof mass, examples of which are discussed above with respect to  FIGS. 11A and 11B . 
     As discussed above, certain electric field detectors, including the electric field detector  200 , may detect an electric field in a one or more dimensions (for example, one or two dimensions of three-dimensional space). In some examples, it may be beneficial to detect aspects of an electric field in multiple dimensions (for example, in two or three dimensions of three-dimensional space). Detecting aspects of an electric field in multiple dimensions may be achieved by implementing multiple electrical field detectors configured to detect an electric field in one direction (also referred to as a one-axis electric field detector) and oriented orthogonally from one another. For example, a sensing system may include three or more one-axis electric field detectors, similar to implementations of the electric field detector  200  being configured to detect aspects of an electric field in one dimension, each oriented orthogonally from one another, such that an electric field is detected in all three dimensions. 
     In other examples, electric field detectors may be configured to detect aspects of an electric field in multiple dimensions. For example, an electric field detector may detect an electric field in two dimensions (also referred to as a two-axis electric field detector), such as certain implementations of the electric field detector  200 . In another example, an electric field detector may detect an electric field in three dimensions (also referred to as a three-axis electric field detector). To detect an electric field in all three dimensions of three-dimensional space, a sensing system may include a two-axis electric field detector and a one-axis electric field detector, two two-axis electric field detectors, a single three-axis electric field detector, or any other combination of electric field detectors. Additional example of two-axis electric field detectors, and examples of three-axis electric field detectors, are provided below. 
       FIG. 14  illustrates a perspective view of an electric field detector  1400  according to an example. The electric field detector  1400  includes a source of concentrated charge  1402 , a proof mass  1404 , a first set of supports  1406  including a first support  1406   a  and a second support  1406   b , a second set of supports  1408  including a third support  1408   a  and a fourth support  1408   b , a first set of anchors  1410  including a first anchor  1410   a  and a second anchor  1410   b , a second set of anchors  1412  including a third anchor  1412   a  and a fourth anchor  1412   b , and a baseplate  1414 . 
     The electric field detector  1400  is substantially similar to the electric field detector  200 . However, rather than having one set of supports  206 , the electric field detector  1400  includes two sets of supports  1406 ,  1408 . The proof mass  1404  may be configured to rotate about two axes, depending on a polarization of the source of concentrated charge  1402 , and may be configured to detect aspects of an electric field in at least two dimensions (for example, the two dimensions of three-dimensional space along which the source of concentrated charge  1402  is not polarized). The additional supports provide a more symmetrical design of the electric field detector  1400 , which facilitates rotation of the electric field detector  1400  in multiple dimensions. For example, the additional supports may suppress movement and/or rotation of the electric field detector  1400  that is not caused predominantly by an external electric field that the electric field detector  1400  is intended to detect. 
     For example, where the source of concentrated charge  1402  is polarized along the z-axis, as illustrated in the example of  FIG. 14 , the electric field detector  1400  may be configured to detect aspects of an electric field in the x-axis (for example, based on rotation of the proof mass  1404  about the y-axis) and aspects of the electric field in the y-axis (for example, based on rotation of the proof mass  1404  about the x-axis). In another example, where the source of concentrated charge  1402  is polarized along the x-axis, the electric field detector  1400  may be configured to detect aspects of an electric field in the y-axis (for example, based on rotation of the proof mass  1404  about the z-axis) and aspects of the electric field in the z-axis (for example, based on rotation of the proof mass  1404  about the y-axis). In another example, where the source of concentrated charge  1402  is polarized along the y-axis, the electric field detector  1400  may be configured to detect aspects of an electric field in the x-axis (for example, based on rotation of the proof mass  1404  about the z-axis) and aspects of the electric field in the z-axis (for example, based on rotation of the proof mass  1404  about the x-axis). Similar to the electric field detector  200 , torsional movement of the proof mass  1404  may be detected based on variations in capacitance between the proof mass  1404  and one or more sense electrodes. 
     Accordingly, the electric field detector  1400  may be particularly well-suited to determine aspects of an electric field in multiple (for example, two) dimensions. A polarization of the source of concentrated charge  1402  may be selected to determine which aspects of the electric field that the electric field detector  1400  determines. In some examples, multiple implementations of the electric field detector  1400  may be implemented together. For example, a first example of the electric field detector  1400  may be implemented in which the source of concentrated charge  1402  is polarized along a first axis, and a second example of the electric field detector  1400  may be implemented in which the source of concentrated charge  1402  is polarized along a second axis, orthogonal to the first axis. If both of these two example detectors are implemented together, then all three orthogonal axes of an electric field may be detected, with one dimension being redundantly determined by both detectors (more particularly, a dimension of the electric field along the axis that is orthogonal to both the first axis and the second axis). 
     As discussed above with respect to  FIGS. 11A and 11B , in some examples, a source of concentrated charge may be replaced by a dielectric material coupled to one or more electrodes to form a dynamic electric dipole. For example, with reference to the electric field detector  1400 , the source of concentrated charge  1402  may be replaced by a dielectric material coupled to one or more electrodes to form a dynamic electric dipole, as discussed with respect to  FIG. 15 . 
       FIG. 15  illustrates a perspective view of a monolithic electric field detector  1500  according to an example. The electric field detector  1500  includes a dielectric material  1502 , a proof mass  1504 , a first set of supports  1506  including a first support  1506   a  and a second support  1506   b , a second set of supports  1508  including a third support  1508   a  and a fourth support  1508   b , a first set of anchors  1510  including a first anchor  1510   a  and a second anchor  1510   b , a second set of anchors  1512  including a third anchor  1512   a  and a fourth anchor  1512   b , a baseplate  1514 , a first set of electrodes  1516  (also referred to herein as a first set of “polarization electrodes”) including a first electrode  1516   a  and a second electrode  1516   b , a first set of traces  1518  including a first trace  1518   a  and a second trace  1518   b , a second set of electrodes  1520  (also referred to herein as a second set of polarization electrodes) including a third electrode  1520   a  and a fourth electrode  1520   b , and a second set of traces  1522  including a third trace  1522   a  and a fourth trace  1522   b.    
     The electric field detector  1500  may include one or more power sources (not illustrated) and/or one or more control circuits (not illustrated). The power source(s) may be coupled to each of the traces  1518 ,  1522  to apply a respective voltage to each of the electrodes  1516 ,  1520 . For example, the control circuit(s) may control the power source(s) to apply a positive voltage (relative to a reference voltage, such as ground) to one of the electrodes  1516   a ,  1516   b , and a negative voltage (relative to the reference voltage) to the other of the electrodes  1516   a ,  1516   b  to generate a potential difference between the electrodes  1516   a ,  1516   b  and thereby polarize the dielectric material  1502  along the x-axis (also referred to herein as a “first polarization axis”). Similarly, the control circuit(s) may control the power source(s) to apply a positive voltage (relative to a reference voltage, such as ground) to one of the electrodes  1520   a ,  1520   b , and a negative voltage (relative to the reference voltage) to the other of the electrodes  1520   a ,  1520   b  to generate a potential difference between the electrodes  1520   a ,  1520   b  and thereby polarize the dielectric material  1502  along the y-axis (also referred to herein as a “second polarization axis”). 
     Accordingly, the control circuit(s) may control the power source(s) to polarize the dielectric material  1502  along either or both of the x-axis and the y-axis. When the dielectric material  1502  is polarized along the x-axis by the electrodes  1516 , a y-component and a z-component of an electric field may be determined based on rotation of the proof mass  1504  about the z-axis and the y-axis, respectively. Similarly, when the dielectric material  1502  is polarized along the y-axis by the electrodes  1520 , an x-component and a z-component of the electric field may be determined based on rotation of the proof mass  1504  about the z-axis and the x-axis, respectively. Thus, by selectively polarizing the dielectric material  1502  in multiple axes, the monolithic electric field detector  1500  is capable of determining aspects of an electric field in all three dimensions of three-dimensional space. 
     More particularly, a polarization of the dielectric material  1502  by the electrodes  1516  along the x-axis may be expressed as, 
         p   x =( V   a   −V   b )*sin( f   1   *t ) 
     where p x  is a polarization of the dielectric material  1502  along the x-axis, V a  is a voltage of the first electrode  1516   a , V b  is a voltage of the second electrode  1516   b , f 1  is a frequency of a voltage provided to the electrodes  1516  by the power source(s), and t is time. Based on this, aspects of an electric field may be determined as, 
         E   y =τ z   /p   x  
 
       and 
         E   z =τ y   /p   x  
 
     where E y  is a y-component of the electric field, τ z  is a torque of the proof mass  1504  about the z-axis, E z  is a z-component of the electric field, and τ y  is a torque of the proof mass  1504  about the y-axis, the torques being determined based on measurements from sensors, such as capacitance sensors, as discussed above. For example, the baseplate  1514  may be coupled to one or more sets of one or more capacitors (not illustrated) configured to sense a change in capacitance resulting from torque of the proof mass  1504 . Accordingly, a y- and z-component of an electric field may be determined based on the polarization of the dielectric material  1502  along the x-axis by the power source(s) and/or control circuit(s). 
     Similarly, a polarization of the dielectric material  1502  by the electrodes  1520  along the y-axis may be expressed as, 
         p   y =( V   c   −V   d )*sin( f   2   *t ) 
     where p y  is a polarization of the dielectric material  1502  along the y-axis, V c  is a voltage of the third electrode  1520   a , V d  is a voltage of the fourth electrode  1520   b, f   2  is a frequency of a voltage provided to the electrodes  1520  by the power source(s), and t is time. Based on this, aspects of an electric field may be determined as, 
         E   x =τ z   /p   y  
 
       and 
         E   z =τ x   /p   y  
 
     where E x  is an x-component of the electric field, τ z  is a torque of the proof mass  1504  about the z-axis, E z  is a z-component of the electric field, and τ x  is a torque of the proof mass  1504  about the x-axis, the torques being determined based on measurements from sensors, such as capacitance sensors, as discussed above. For example, the baseplate  1514  may be coupled to one or more sets of one or more capacitors (not illustrated) configured to sense a change in capacitance resulting from torque of the proof mass  1504 . Accordingly, an x- and z-component of an electric field may be determined based the polarization of the dielectric material  1502  along the y-axis by the power source(s) and/or control circuit(s). 
     Thus, the electric field detector  1500  may be configured to detect aspects of an electric field in all three dimensions of three-dimensional space. In the example provided above, the electric field detector  1500  detects an x-, y-, and z-component of an electric field, including redundantly detecting the z-component of the electric field based on both polarizations of the dielectric material  1502 . In other examples, the electric field detector  1500  may include additional electrodes to polarize the dielectric material  1502  along the z-axis as well, in addition to or in lieu of the electrodes  1516 ,  1520 . That is, in some examples, the electric field detector  1500  may include any combination of electrodes to polarize the dielectric material  1502  in any number and combination of dimensions, such that the electric field detector  1500  may detect aspects of an electric field in any number and combination of dimensions. 
     As discussed above, the electrodes  1516 ,  1520  may be driven by power source(s) and/or controller(s) at respective AC frequencies f 1 , f 2 . For example, the AC frequencies f 1 , f 2  may range from approximately 20 kHz to approximately 1 MHz in some examples. In some examples, the frequencies f 1 , f 2  are different from one another such that the electrodes  1516 ,  1520  may be simultaneously polarize the dielectric material  1502  in two dimensions, with the electric field components E y , E z  being up-converted to frequency f 1  and the electric field components E x , E z  being up-converted to frequency f 2 . In this manner, the electric field components E y , E z  may be differentiated from the electric field components E x , E z  because they correspond to (for example, are up-converted to) the different frequencies f 1 , f 2 . The electric field components may subsequently be separately identified by de-modulating the electric field components to identify a baseband signal. A frequency of the baseband signal (that is, one of frequencies f 1 , f 2 ) is recovered to associate the correct electric field components with the recovered baseband signal frequency. Thus, the frequencies f 1 , f 2  may be differentiated to uniquely identify one or more dimensions associated with the electric field components. 
     As such, in addition to providing improved electric field detectors that exploit the electric component of electromagnetic signals, various other aspects and examples discussed herein provide improved fabrication processes for efficiently and cost-effectively producing a compact electric field detector. Particular examples of the electric field detector may include an electric field detector capable of detecting bio-physical signals generated by the body of a patient or user, such as the electric field of his or her brain, heart, nerves or muscles. When compared to available electromagnetic sensors examples of the electric field detector herein achieve a low noise (for example, less than 1 mV/m/rtHz at 10 Hz) at a compact size (for example, less than 1 cm 3 ) and a low production cost. 
     As discussed above, in some embodiments, movement of a proof mass (for example, any of the proof masses  202 ,  1102 ,  1404 ,  1504 ) may be determined based on one or more capacitive sensors. In other examples, other sensors may be implemented to determine movement of a proof mass in addition to or in lieu of the capacitance sensors. For example, an optical sensor may be implemented to optically determine movement of the proof mass, and determine parameters of an electric field therefrom. In another example, a resistive sensor may be implemented having a resistance that varies based on movement of the proof mass. Variations in the resistance of the resistive sensor may be determined (for example, by identifying variations in a signal provided to the resistive sensor and determining variations in the resistance of the resistive sensor therefrom), and parameters of an electric field generating the variations in the resistance of the resistive sensor may be determined therefrom. 
     Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the disclosure. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the disclosure should be determined from proper construction of the appended claims, and their equivalents.