Patent Publication Number: US-2022234762-A1

Title: Capacitance sensing instruments and methods for use

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a non-provisional patent application claiming priority to U.S. Provisional Application No. 63/142,739, filed on Jan. 28, 2021, the contents of which are hereby incorporated by reference. 
    
    
     FIELD 
     The present disclosure generally relates to sensing instruments and methods for using them, and more specifically to sensing instruments and methods for sensing a capacitance of a material. 
     BACKGROUND 
     Some aircraft include structural components that are made of aluminum and/or alloys that contain aluminum. Many of these structural components are regularly exposed to ambient environmental conditions such as wind and rain. As such, these structural components tend to exhibit corrosion as time passes, especially at areas like rivet joints. Often, the outer surfaces of these structural components are covered by a layer of paint and/or one or more passivation layers, which can make detection of the corrosion difficult. For example, paint and/or passivation layers can be removed abrasively or via other means, and the corrosion can be detected by visual inspection. However, this is a time-consuming and labor intensive process. As such, a need exists for instruments and methods that facilitate less invasive and quicker detection of corrosion in structural components. 
     SUMMARY 
     One aspect of the disclosure is a sensing instrument comprising: a first electrode, a second electrode that surrounds the first electrode, and a sensing module configured to sense a capacitance of a material by applying a voltage between the first electrode and the second electrode while the first electrode and the second electrode are adjacent to the material. 
     Another aspect of the disclosure is a method of operating a sensing instrument, the method comprising: applying a voltage between a first electrode and a second electrode while the first electrode and the second electrode are positioned adjacent to a material, wherein the second electrode surrounds the first electrode, and sensing a capacitance of the material based on a response of the material to the voltage. 
     Yet another aspect of the disclosure is a method of operating a sensing instrument, the method comprising: applying a first voltage between a first electrode and a second electrode while the first electrode and the second electrode are positioned adjacent to a first region of a material, wherein the second electrode surrounds the first electrode, sensing a first capacitance of the material based on a first response of the material to the first voltage, applying a second voltage between a third electrode and a fourth electrode while the third electrode and the fourth electrode are positioned adjacent to a second region of the material, wherein the fourth electrode surrounds the third electrode, sensing a second capacitance of the material based on a second response of the material to the second voltage, applying a third voltage between a fifth electrode and a sixth electrode while the fifth electrode and the sixth electrode are positioned adjacent to a third region of the material, wherein the sixth electrode surrounds the fifth electrode, and sensing a third capacitance of the material based on a third response of the material to the third voltage. 
     By the term “about” or “substantially” with reference to amounts or measurement values described herein, it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide. 
     Use of broader terms such as comprises, includes, and having should be understood to provide support for narrower terms such as consisting of, consisting essentially of, and comprised substantially of. Use of the term “optionally,” “may,” “might,” “possibly,” and the like with respect to any element of an embodiment means that the element is not required, or alternatively, the element is required, both alternatives being within the scope of the embodiment(s). 
     The use of ordinal numbers such as “first,” “second,” “third,” and so on is meant to distinguish respective elements rather than to denote a particular order or importance of those elements. 
     The features, functions, and advantages that have been discussed can be achieved independently in various examples or can be combined in yet other examples further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the illustrative examples are set forth in the appended claims. The illustrative examples, however, as well as a preferred mode of use, further objectives and descriptions thereof, will best be understood by reference to the following detailed description of an illustrative example of the present disclosure when read in conjunction with the accompanying Figures. 
         FIG. 1  is a block diagram of a sensing instrument, according to an example. 
         FIG. 2  is a block diagram of a sensing module, according to an example. 
         FIG. 3  is a perspective view of an aircraft, according to an example. 
         FIG. 4  is a bottom view of a circuit board and electrodes, according to an example. 
         FIG. 5  is a top view of a circuit board, according to an example. 
         FIG. 6  is a cross sectional view of an electrode set, a material, and a spacer, according to an example. 
         FIG. 7  is a cross sectional view of an electrode set, a material, and a spacer, according to an example. 
         FIG. 8  is a cross sectional diagram of a material and a circuit board, according to an example. 
         FIG. 9  is a cross sectional diagram of a material and a circuit board, according to an example. 
         FIG. 10  is a top view of regions of a material, according to an example. 
         FIG. 11  is a bottom view of a row of electrode sets of a sensing instrument on a circuit board, according to an example. 
         FIG. 12  is a cross-sectional view of a row of electrode sets of a sensing instrument and a material, according to an example. 
         FIG. 13  is a bottom view of a row of electrode sets of a sensing instrument on a circuit board, according to an example. 
         FIG. 14  is a cross-sectional view of a row of electrode sets of a sensing instrument and a material, according to an example. 
         FIG. 15  is a top view of a circuit board and a material, according to an example. 
         FIG. 16  is a top view of a circuit board and a material, according to an example. 
         FIG. 17  shows a display component of a user interface, according to an example. 
         FIG. 18  is a cross sectional view of a circuit board and a material, according to an example. 
         FIG. 19  is a block diagram of a method, according to an example. 
         FIG. 20  is a block diagram of a method, according to an example. 
         FIG. 21  is a block diagram of a method, according to an example. 
         FIG. 22  is a block diagram of a method, according to an example. 
         FIG. 23  is a block diagram of a method, according to an example. 
         FIG. 24  is a block diagram of a method, according to an example. 
         FIG. 25  is a block diagram of a method, according to an example. 
         FIG. 26  is a block diagram of a method, according to an example. 
     
    
    
     DETAILED DESCRIPTION 
     As discussed above, a need exists for instruments and methods that facilitate less invasive and quicker detection of corrosion in structural components. Often, the outer surfaces of these structural components are covered by a layer of paint and/or one or more passivation layers. Unlike conventional instruments and methods, the instruments and methods disclosed herein can be used to detect corrosion without removal of the paint or passivation layers. 
     Examples disclosed herein include a sensing instrument that includes a first electrode, a second electrode that surrounds the first electrode, and a sensing module configured to sense a capacitance of a material by applying a voltage between the first electrode and the second electrode while the first electrode and the second electrode are adjacent to the material. 
     Additional examples disclosed herein include a method of operating a sensing instrument that includes applying a voltage between a first electrode and a second electrode while the first electrode and the second electrode are positioned adjacent to a material. The second electrode surrounds the first electrode. The method further includes sensing a capacitance of the material based on a response of the material to the voltage. 
     Additionally or alternatively, the first electrode is coplanar with the second electrode and/or the first electrode and the second electrode take the form of concentric rings that are substantially parallel with an outer surface of the material under test. Generally, the first electrode and the second electrode do not make contact with the material (e.g., an aircraft wing skin) during operation, but the electric field formed between the first electrode and the second electrode penetrates into the material. The sensing module can include a signal generator configured to apply the voltage between the first electrode and the second electrode and a meter that is configured to sense a voltage response and/or a current response of the material to the applied voltage. The capacitance of the material adjacent to the first electrode and the second electrode can be derived based on the response. As the instrument is used to test different areas of the material, variances in capacitance sensed by the sensing module can be inferred to indicate areas of corrosion. 
     Additionally or alternatively, one-dimensional or two-dimensional arrays of electrode sets are formed. That is, several first electrodes and corresponding second electrodes form a row of electrode sets and/or multiple rows and columns of electrode sets. These instruments having multiple sets of (e.g., concentric and coplanar) electrodes are used to detect capacitances of the material at many different locations within the material (e.g., simultaneously). This capacitance data collected over a line or over an area of the material can be used to generate an “image” where pixel intensity or color is mapped to capacitance levels. 
     The aforementioned instruments and methods can be advantageous when compared to conventional instruments and methods because the aforementioned instruments and methods can involve less invasive and quicker detection of corrosion in various materials. For example, corrosion detection can be performed without removing paint or passivation layers from a surface of the material under test and without having to reapply the paint or passivation layers. The aforementioned instruments and methods can also be used to detect corrosion or other anomalies in materials that are not covered by paint or passivation layers. 
     Disclosed examples will now be described more fully hereinafter with reference to the accompanying Drawings, in which some, but not all of the disclosed examples are shown. Indeed, several different examples are described and should not be construed as limited to the examples set forth herein. Rather, these examples are described so that this disclosure will be thorough and complete and will fully convey the scope of the disclosure to those skilled in the art. 
       FIGS. 1-18  depict components of and functionality related to an aircraft, a sensing instrument, and/or a material under test. 
       FIG. 1  is a block diagram of a sensing instrument  100 . The sensing instrument  100  includes a first electrode  102 , a second electrode  104  that surrounds the first electrode  102 , and a sensing module  106 . As described in more detail below, the sensing module  106  is configured to sense a capacitance  108  of a material  110  by applying a voltage  112  between the first electrode  102  and the second electrode  104  while the first electrode  102  and the second electrode  104  are adjacent to the material  110 . The first electrode  102 , the second electrode  104 , and perhaps other electrodes are collectively referred to as an electrode set  119 A. The sensing instrument  100  also includes a circuit board  116  on which the first electrode  102 , the second electrode  104 , and perhaps other electrodes are positioned. 
       FIG. 2  is a block diagram of the sensing module  106 . The sensing module  106  includes one or more processors  222 , a non-transitory computer readable medium  224 , a communication interface  226 , a user interface  230 , a meter  236 , and a signal generator  238 . Components of the sensing module  106  are linked together by a system bus, network, or other connection mechanism  232 . 
     The one or more processors  222  can be any type of processor(s), such as a microprocessor, a digital signal processor, a multicore processor, etc., coupled to the non-transitory computer readable medium  224 . 
     The non-transitory computer readable medium  224  can be any type of memory, such as volatile memory like random access memory (RAM), dynamic random access memory (DRAM), static random access memory (SRAM), or non-volatile memory like read-only memory (ROM), flash memory, magnetic or optical disks, or compact-disc read-only memory (CD-ROM), among other devices used to store data or programs on a temporary or permanent basis. 
     Additionally, the non-transitory computer readable medium  224  stores instructions  234 . The instructions  234  are executable by the one or more processors  222  to cause the sensing module  106  to perform any of the functions or methods described herein. 
     The communication interface  226  includes hardware to enable communication within the sensing module  106  and/or between the sensing module  106  and one or more other devices. The hardware includes transmitters  252 , receivers  254 , and antennas  256 , for example. The communication interface  226  is configured to facilitate communication with one or more other devices, in accordance with one or more wired or wireless communication protocols. For example, the communication interface  226  is configured to facilitate wireless data communication for the sensing module  106  according to one or more wireless communication standards, such as one or more Institute of Electrical and Electronics Engineers (IEEE) 801.11 standards, ZigBee standards, Bluetooth standards, etc. As another example, the communication interface  226  is configured to facilitate wired data communication with one or more other devices. 
     The user interface  230  includes one or more pieces of hardware used to provide data and control signals to the sensing module  106 . For instance, the user interface  230  can include a mouse or a pointing device, a keyboard or a keypad, a microphone, a touchpad, or a touchscreen, among other possible types of user input devices. Generally, the user interface  230  enables an operator to interact with a graphical user interface (GUI) provided by the sensing module  106 . The user interface  230  generally includes a display component configured to display data. As one example, the user interface  230  includes a touchscreen display. As another example, the user interface  230  includes a flat-panel display, such as a liquid-crystal display (LCD) or a light-emitting diode (LED) display. 
     The sensing module  106  also includes the meter  236  (e.g., a multimeter). The meter  236  includes a first port  240  and a second port  242 . The first port  240  is configured to receive an electrical cable or another electrical connector that connects the first electrode  102  to the first port  240  and the second port  242  is configured to receive an electrical cable or another electrical connector that connects the second electrode  104  to the second port  242 . The meter  236  is configured to sense steady state and transient voltages present between the first port  240  and the second port  242 . The meter  236  is also configured to sense steady state and transient currents that flow from the first port  240  to the second port  242 . 
     The sensing module  106  also includes the signal generator  238 . The signal generator  238  includes a third port  244  and a fourth port  246 . The third port  244  is configured to receive an electrical cable or another electrical connector that connects the first electrode  102  to the third port  244  and the fourth port  246  is configured to receive an electrical cable or another electrical connector that connects the second electrode  104  to the fourth port  246 . The signal generator  238  is configured to generate AC or DC voltage or current signals between the third port  244  and the fourth port  246  to induce a steady state and/or transient voltage and/or current response within a material under test. The response of the material is sensed by the meter  236  and used to determine a capacitance of the material, for example. 
       FIG. 3  is a perspective view of an aircraft  700 . The aircraft  700  includes a nose  710 , a wing  720   a , a wing  720   b , a fuselage  725 , and a tail  730 . The aircraft  700  includes many areas arranged for storage of items during flight. In one example, the fuselage  725  includes storage underneath a passenger compartment for storing luggage and other items or supplies. In another example, the passenger compartment in the fuselage  725  includes overhead bins and under seat areas for storing further items. 
     Additionally or alternatively, the sensing instrument  100  is used to determine capacitances of various areas of structural components (e.g., skins) that form various components of the aircraft  700 . Capacitances that vary significantly from a baseline typically are inferred to indicate an area of corrosion (e.g., a subsurface area of corrosion) because changes in capacitance generally indicate a change in material composition. 
     Although an aircraft is used as an example herein, the sensing instrument  100  can also be used to measure the capacitance of and/or detect material anomalies within other structures such as buildings, bridges, boats, ships, and railcars, or other vehicles. 
       FIG. 4  is a bottom view of the circuit board  116  and electrodes. The electrode set  119 A includes the first electrode  102 , the second electrode  104 , and a third electrode  114 . The first electrode  102 , the second electrode  104 , and the third electrode  114  are electrically conductive (e.g., metallic) and are printed or otherwise formed on a first side  118  (e.g., a bottom side) of the circuit board  116 . 
     The first electrode  102  has a circular shape, but other examples are possible. The second electrode  104  and the third electrode  114  each have an annular shape, but other examples are possible. The second electrode  104  surrounds the first electrode  102  and the third electrode  114  surrounds both the second electrode  104  and the first electrode  102 . The first electrode  102 , the second electrode  104 , and the third electrode  114  have respective thicknesses ranging from 1 μm to 1 mm measured normal to the first side  118 . Thicknesses of the first electrode  102 , the second electrode  104 , and the third electrode  114  are generally substantially equal to each other. 
     The first electrode  102  and the second electrode  104  together have reflectional symmetry and rotational symmetry. As such, the first electrode  102 , the second electrode  104 , and the third electrode  114  also together have rotational and reflectional symmetry. The first electrode  102 , the second electrode  104 , and the third electrode  114  are also coplanar. The rotational symmetry, the reflectional symmetry, and the electrodes being coplanar can help to eliminate undesirable electric field fringing during use. 
     The circuit board  116  can be a printed circuit board (PCB) or another type of circuit board. Generally, the circuit board  116  will have an electrically insulating core with conductive circuitry printed thereon. 
       FIG. 5  is a top view of the circuit board  116 . The circuit board  116  includes a metal shielding layer  120  on a second side  122  of the circuit board  116  that is opposite the first side  118 . The metal shielding layer  120  can help isolate the circuit board  116  from external electric fields. 
       FIG. 6  is a cross sectional view of the electrode set  119 A, the material  110 , and a spacer  124 . In  FIG. 6 , the spacer  124  takes the form of a piece of electrically insulating material, such as foam or another low-κ dielectric material (e.g., 1.0&lt;κ&lt;3.0). For ease of use, the spacer  124  will typically be formed of lightweight materials. The spacer  124  maintains a minimum distance  126  (e.g., substantially equal to a thickness of the spacer  124 ) between (i) the first electrode  102  or the second electrode  104  and (ii) the material  110 . The minimum distance  126  could range from 0.05 mm to 3 mm, but other examples are possible. The minimum distance  126  is generally selected such that electric fields that are generated between the first electrode  102  and the second electrode  104  penetrate into the material  110  without the material  110  creating a short circuit between the first electrode  102  and the second electrode  104 . The material  110  is a portion of the nose  710 , the wing  720   a , the wing  720   b , the fuselage  725 , or the tail  730  of the aircraft  700 , for example. Additionally or alternatively, the minimum distance  126  could be implemented as approximately equal to the diameter d 3  of the electrode set  119 A (see  FIG. 16 ). 
       FIG. 7  is a cross sectional view of the electrode set  119 A, the material  110 , and another embodiment of the spacer  124 . In  FIG. 7 , the spacer  124  includes a platform  125  and three studs  123 . The studs  123  have equal depths corresponding to the vertical direction in  FIG. 7 . The equal depths define a plane. The depth of the stud  123  added to a depth of the platform  125  is equal to the minimum distance  126 . The studs  123  can be formed of plastic, but other examples are possible. In some examples, the studs  123  are replaced with wheels that have equal thicknesses. The three studs  123  define a plane such that the electrode set  119 A can be maintained a substantially constant distance (e.g., the minimum distance  126 ) from the material  110 , which helps maintain the accuracy and/or consistency of the capacitance measurements. 
       FIG. 8  is a cross sectional diagram of the material  110  and the circuit board  116 . The signal generator  238  of the sensing instrument  100  applies a voltage  112  (e.g., AC and/or DC) between the first electrode  102  and the second electrode  104  while the first electrode  102  and the second electrode  104  are positioned adjacent to the material  110 . As shown, the second electrode  104  surrounds the first electrode  102 . 
     The first electrode  102  and the second electrode  104  being “adjacent” to the material  110  can mean that the first electrode  102  and the second electrode  104  are close enough to the material  110 , based on the voltage  112  and the geometry of the first electrode  102  and the second electrode  104 , to have a significant portion of an electric field  113  generated by the voltage  112  penetrate the material  110 . For example, “adjacent” will generally mean that the first electrode  102  and the second electrode  104  are close enough to the material  110  (e.g., but not in contact) to accurately sense a capacitance of the material  110 . In some examples, the term “adjacent” refers to a range of separation between the material  110  and the first electrode  102  or the second electrode  104  of 0.05 mm to 3 mm, but other examples are possible. One of ordinary skill in the art would be able to determine a suitable distance between the material  110  and the first electrode  102  and/or the second electrode  104 , based on the voltage  112  and the geometry of the first electrode  102  and/or the second electrode  104 . 
     Next, the meter  236  senses the capacitance  108  of the material  110  based on a response V 1  of the material  110  to the voltage  112 . The response V 1  is generally sensed by the meter  236  between the first electrode  102  and the second electrode  104 , but other examples are possible. The response V 1  takes the form of a transient or steady state voltage or current, for example, having a particular amplitude or magnitude and/or having a particular decay constant or phase relative to the voltage  112 . The amplitude, the magnitude, the decay constant, and/or the phase is used to determine the capacitance  108 . It should be noted that the signal generator  238  has a finite series resistance that will typically cause the response V 1  to be different from the voltage  112 , because some voltage produced by the signal generator  238  is dropped across that series resistance and not entirely between the first electrode  102  and the second electrode  104 . 
       FIG. 9  is a cross sectional diagram of the material  110  and a version of the circuit board  116  that includes the third electrode  114 . The signal generator  238  applies a second voltage  135  between the first electrode  102  and the third electrode  114  concurrently with applying the first voltage  112  between the first electrode  102  and the second electrode  104 . The second voltage  135  is applied while the third electrode  114  is positioned adjacent to the material  110  as well. As shown, the third electrode  114  surrounds the second electrode  104 . The signal generator  238  applying the second voltage  135  will generally shape the electric field  113  formed between the first electrode  102  and the second electrode  104 . For example, changing the magnitude and/or the polarity of the second voltage  135  typically varies a depth a significant portion of the electric field  113  penetrates into the material  110 . Varying the depth of the electric field  113  allows for taking capacitance measurements of or sensing anomalies within varying depths of the material  110 . 
     It should also be noted that, the first voltage  112  and the second voltage  135  can be generated in ways other than those depicted in  FIG. 9 . For example, the first voltage  112  and the second voltage  135  could both have a negative terminal at a common ground connection. Other examples are possible. Next, the meter  236  senses the capacitance  108  of the material  110  as described above with reference to  FIG. 8 . However, the second voltage  135  generally has an impact on the capacitance  108  detected by the meter  236 , for example, by changing a volume of the material  110  that the capacitance  108  actually represents (e.g., by changing the shape of the electric field  113 ). 
       FIG. 10  is a top view of the material  110 , depicting regions  150 ,  152 ,  154 ,  156 ,  158 ,  190 ,  192 ,  194 ,  196 , and  198  of the material  110 . As described below with reference to  FIGS. 11-14 , the sensing instrument  100  is used to determine respective capacitances corresponding to the regions  150 ,  152 ,  154 ,  156 ,  158 ,  190 ,  192 ,  194 ,  196 , and  198  of the material  110 . 
       FIG. 11  is a bottom view of a row of electrode sets  119 A,  119 B,  119 C,  119 D, and  119 E of the sensing instrument  100  on the circuit board  116 . Although five electrode sets are shown in  FIG. 11 , any number of electrode sets could be included as part of the circuit board  116 . Additional electrode sets provide additional elements of independent control during testing of a material and also can reduce an amount of time needed to inspect a given area and/or volume of material. 
     The sensing instrument  100  includes the electrode set  119 B on the first side  118  of the circuit board  116 . The electrode set  119 B includes a third electrode  128  and a fourth electrode  130  that surrounds the third electrode  128 . 
     The sensing instrument  100  also includes the electrode set  119 C on the first side  118  of the circuit board  116 . The electrode set  119 C includes a fifth electrode  132  and a sixth electrode  134  that surrounds the fifth electrode  132 . 
     The sensing instrument  100  also includes the electrode set  119 D and the electrode set  119 E which both generally have all of the features of the electrode sets  119 A-C (e.g., two or three concentric and/or coplanar electrodes). 
     As shown, the first electrode  102 , the third electrode  128 , and the fifth electrode  132  are collinear. Respective center points of the second electrode  104 , the fourth electrode  130 , and the sixth electrode  134  are also collinear. As such, electrode sets  119 A,  119 B,  119 C,  119 D, and  119 E form a row that can be scanned over the material  110  to take capacitance measurements over a two-dimensional area. 
     The signal generator  238  applies a second voltage  136  between the third electrode  128  and the fourth electrode  130  while the third electrode  128  and the fourth electrode  130  are adjacent to the material  110  (not shown in  FIG. 11 ). The signal generator  238  applies the second voltage  136  concurrent with applying the first voltage  112 , for example. 
     The signal generator  238  also applies a third voltage  138  between the fifth electrode  132  and the sixth electrode  134  while the fifth electrode  132  and the sixth electrode  134  are adjacent to the material  110  (not shown in  FIG. 11 ). The aforementioned voltages applied between electrodes are used to measure capacitance of the material  110  underneath the respective electrodes. 
       FIG. 12  is a cross-sectional view of the row of electrode sets  119 A,  119 B,  119 C,  119 D, and  119 E of the sensing instrument  100  on the circuit board  116 . 
     The signal generator  238  applies the first voltage  112  between the first electrode  102  and the second electrode  104  while the first electrode  102  and the second electrode  104  are positioned adjacent to a first region  150  of the material  110 . As shown, the second electrode  104  surrounds the first electrode  102 . The circuit board  116  is positioned manually or automatically such that the first electrode  102  and the second electrode  104  are positioned adjacent to the first region  150 . 
     The meter  236  senses the first capacitance  108  of the material  110  (e.g., the first region  150 ) based on the first response V 1  of the material  110  to the first voltage  112 . 
     The signal generator  238  applies the second voltage  136  between the third electrode  128  and the fourth electrode  130  while the third electrode  128  and the fourth electrode  130  are positioned adjacent to a second region  152  of the material  110 . As shown, the fourth electrode  130  surrounds the third electrode  128 . Once the circuit board  116  is positioned such that the first electrode  102  and the second electrode  104  are positioned adjacent to the first region  150 , the circuit board  116  will also be positioned such the third electrode  128  and the fourth electrode  130  are adjacent to the second region  152 . 
     The meter  236  senses a second capacitance  161  of the material  110  (e.g., the second region  152 ) based on a second response V 2  of the material  110  to the second voltage  136 . 
     The signal generator  238  applies the third voltage  138  between the fifth electrode  132  and the sixth electrode  134  while the fifth electrode  132  and the sixth electrode  134  are positioned adjacent to a third region  154  of the material  110 . As shown, the sixth electrode  134  surrounds the fifth electrode  132 . Once the circuit board  116  is positioned such that the first electrode  102  and the second electrode  104  are positioned adjacent to the first region  150 , the circuit board  116  will also inherently be positioned such the fifth electrode  132  and the sixth electrode  134  are adjacent to the third region  154 . 
     The meter  236  senses a third capacitance  163  of the material  110  (e.g., the third region  154 ) based on a third response V 3  of the material  110  to the third voltage  138 . 
     The signal generator  238  applies the first voltage  112 , the second voltage  136 , and the third voltage  138  simultaneously, but other examples are possible. Generally, the first voltage  112 , the second voltage  136 , and the third voltage  138  are substantially equal in magnitude, timing, phase, and/or waveform. However, there may be situations where the first voltage  112 , the second voltage  136 , and/or the third voltage  138  having different amplitudes, timing, phase, and/or waveforms could be beneficial. 
       FIG. 13  is a bottom view of the row of electrode sets  119 A,  119 B,  119 C,  119 D, and  119 E of the sensing instrument  100  on the circuit board  116 . 
       FIG. 14  is a cross-sectional view of the row of electrode sets  119 A,  119 B,  119 C,  119 D, and  119 E of the sensing instrument  100  on the circuit board  116 . In  FIG. 14 , the circuit board  116  has been moved manually or automatically to a position relative to the material  110  that is different from the position depicted in  FIG. 12 . 
     The signal generator  238  applies a fourth voltage  512  between the first electrode  102  and the second electrode  104  while the first electrode  102  and the second electrode  104  are positioned adjacent to a fourth region  190  of the material  110 . The fourth voltage  512  can be the same as the first voltage  112 , but it is not required. 
     The meter  236  senses a fourth capacitance  508  of the material  110  (e.g., the fourth region  190 ) based on a fourth response V 4  of the material  110  to the fourth voltage  512 . 
     The signal generator  238  applies a fifth voltage  536  between the third electrode  128  and the fourth electrode  130  while the third electrode  128  and the fourth electrode  130  are positioned adjacent to a fifth region  192  of the material  110 . Once the circuit board  116  is positioned such that the first electrode  102  and the second electrode  104  are positioned adjacent to the fourth region  190 , the circuit board  116  will also be positioned such the third electrode  128  and the fourth electrode  130  are adjacent to the fifth region  192 . 
     The meter  236  senses a fifth capacitance  561  of the material  110  (e.g., the fifth region  192 ) based on a fifth response V 5  of the material  110  to the fifth voltage  536 . 
     The signal generator  238  applies a sixth voltage  538  between the fifth electrode  132  and the sixth electrode  134  while the fifth electrode  132  and the sixth electrode  134  are positioned adjacent to a sixth region  194  of the material  110 . Once the circuit board  116  is positioned such that the first electrode  102  and the second electrode  104  are positioned adjacent to the fourth region  190 , the circuit board  116  will also be positioned such the fifth electrode  132  and the sixth electrode  134  are adjacent to the sixth region  194 . 
     The meter  236  senses a sixth capacitance  563  of the material  110  (e.g., the sixth region  194 ) based on a sixth response V 6  of the material  110  to the sixth voltage  538 . 
     The signal generator  238  applies the fourth voltage  512 , the fifth voltage  536 , and the sixth voltage  538  simultaneously, but this is not necessary. 
     The fifth voltage  536  and the sixth voltage  538  can be the same as the fourth voltage  512 , but it is not necessary. 
       FIG. 15  is a top view of the circuit board  116  and the material  110 . The circuit board  116  is moved or scanned automatically or manually in a direction  159  to systematically examine various regions of the material  110  using the electrode sets  119 A,  119 B,  119 C,  119 D, and  119 E. During or prior to such a scan, the sensing module  106  can determine that a baseline capacitance C b  represents an average expected capacitance of the material  110  in the absence of substantial anomalies such as corrosion. In one example, the electrode set  119 A is eventually positioned over an anomaly  146  of the material  110  (e.g., an area of corrosion). The sensing module  106  determines that the capacitance  108  of the material  110  (e.g., the anomaly  146 ) differs from the baseline capacitance C b  by more than a threshold difference C t . In  FIG. 15 , the capacitance  108  is represented by C xy . In some examples, the threshold difference C t  could be 1%, 2%, 3%, 5%, 10%, or 15% of the baseline capacitance C b . The threshold difference C t  is selected to represent a variance in capacitance that is great enough to lend some degree of certainty that an anomaly has been detected. One of ordinary skill in the art would recognize (e.g., via diagnostic testing) how to select a threshold difference that accurately represents an anomaly in the material  110 . 
     In response to the sensing module  106  determining that the capacitance  108  (e.g., C xy ) of the material  110  (e.g., the anomaly  146 ) differs from the baseline capacitance C b  by more than the threshold difference C t , the user interface  230  provides an indication  144  that the anomaly  146  exists beneath the first electrode  102  and the second electrode  104 . This is shown in  FIG. 17  and discussed in more detail below. 
     After examining the anomaly  146 , the circuit board  116  is moved in a direction  169  such that the electrode set  119 A is over a testing region  148  of the material  110 . While the first electrode  102  and the second electrode  104  are moved over the material  110  from the anomaly  146  to be adjacent to the testing region  148 , the electrode set  119 A (e.g., continuously) senses a capacitance of the material  110 . The sensing module  106  low-pass filters the capacitance sensed while moving the first electrode  102  and the second electrode  104  in the direction  169 . The low-pass filtering helps disregard changes in capacitance that can occur due to the distance between the electrode set  119 A and the material  110  changing slowly while the circuit board  116  is moved in the direction  169 . This change in distance is attributed to user error or to non-idealities of the spacer  124 , for example. The low-pass filtering can allow focus on more abrupt changes in capacitance which more likely represent changes in composition of the material  110 . Gradual changes in capacitance that occur while the circuit board  116  moves can thus be ignored because those gradual changes likely reflect non-idealities of the sensing instrument  100  itself and not changes in the composition of the material  110 . 
     Additionally, the sensing module  106  uses the electrode set  119 C to determine that a capacitance of the material  110  corresponding to an anomaly  147  differs from the baseline capacitance C b  by more than the threshold difference C t  and responsively provide an indication  149  via the user interface  230  that the anomaly  147  exists beneath the electrode set  119 C. This is shown in  FIG. 17  and discussed in more detail below. 
       FIG. 16  is a top view of another embodiment of the circuit board  116  and the material  110 . As shown, the first electrode  102 , the third electrode  128 , and the fifth electrode  132  are not collinear in this embodiment.  FIG. 16  shows a three-dimensional array of electrode sets  119 A,  119 B,  119 C,  119 D,  119 E,  119 F,  119 G,  119 H,  119 I, and  119 J. The three-dimensional array can be useful in quickly inspecting large volumes and/or areas of the material  110 . 
     The circuit board  116  is moved or scanned automatically or manually in a direction  159  to systematically examine various regions of the material  110  using one or more of the electrode sets  119 A,  119 B,  119 C,  119 D,  119 E,  119 F,  119 G,  119 H,  119 I, and  119 J. During or prior to such a scan, the sensing module  106  can determine that a baseline capacitance C b  represents an average expected capacitance of the material  110  in the absence of substantial abnormalities such as corrosion. In one example, the electrode set  119 F is eventually positioned over an anomaly  146  of the material  110  (e.g., an area of corrosion). The sensing module  106  determines that the capacitance  108  of the material  110  (e.g., the anomaly  146 ) differs from the baseline capacitance C b  by more than a threshold difference C t . In  FIG. 16 , the capacitance  108  is represented by C xy . In some examples, the threshold difference C t  could 1%, 2%, 3%, 5%, 10%, or 15% of the baseline capacitance C b , but other examples are possible. The threshold difference C t  is selected to represent a variance in capacitance that is great enough to lend some degree of certainty that an anomaly has been detected. 
     In response to the sensing module  106  determining that the capacitance  108  (e.g., C xy ) of the material  110  (e.g., the anomaly  146 ) differs from the baseline capacitance C b  by more than the threshold difference C t , the user interface  230  provides an indication  144  that the anomaly  146  exists beneath the fifth electrode  132  and the sixth electrode  134 . This is shown in  FIG. 17  and discussed in more detail below. 
     Additionally, the sensing module  106  uses the electrode set  119 C to determine that a capacitance of the material  110  corresponding to the anomaly  147  differs from the baseline capacitance C b  by more than the threshold difference C t  and responsively provide an indication  149  via the user interface  230  that the anomaly  147  exists beneath the electrode set  119 C. This is shown in  FIG. 17  and discussed in more detail below. 
     As shown in  FIG. 16 , the electrode sets  119 A-E are respectively separated from the electrode sets  119 F-J by a distance d 1 . On another axis, the electrode sets (e.g., the electrode set  119 D and the electrode set  119 E) are separated by a distance d 2 . Generally, the distance d 1  and the distance d 2  can be selected and implemented based on the size of the anomalies that are of interest and/or are anticipated. For example, if the anomalies of interest have diameters that are similar in scale to the diameters d 3  of the electrode sets  119 A-J, d 1  and d 2  can be implemented as approximately half of d 3 . (d 1 , d 2 , and d 3  are not necessarily shown to scale in  FIG. 16 .) In examples in which the third electrode  114  is used to electromagnetically isolate the electrode set  119 A from the other electrode sets, the electrode sets  119 A-J could nearly abut each other. 
       FIG. 17  shows a display component of the user interface  230 . As discussed above, the user interface  230  provides the indication  144  that the anomaly  146  exists beneath the first electrode  102  and the second electrode  104 . For example, the indication  144  can include darkening, lightening, or changing a color of a region of the display component that corresponds to the anomaly  146 . In a similar fashion, the user interface  230  also provides the indication  149  that the anomaly  147  exists beneath the electrode set  119 C or the electrode set  119 H. 
     The capacitance corresponding to the anomaly  147  might differ less from the baseline capacitance C b  when compared to the capacitance corresponding to the anomaly  146 . As such, the indication  149  might indicate that via a difference in brightness or color when compared to the indication  144 . For example, lower pixel intensity could correlate with a higher variance from the baseline capacitance, or a color scale could be mapped to different levels of difference from the baseline capacitance C b . In some examples, the actual capacitance values could be displayed at each respective region within the user interface  230 . Other examples are possible. These concepts are also applied three-dimensionally to achieve a three dimensional mapping of abnormalities within the material  110 . Such mappings can be obtained periodically over time to identify trends in deterioration of the material  110 . 
       FIG. 18  is a cross sectional view of the circuit board  116  and the material  110 . The sensing module  106  determines that the first capacitance  108  of the material  110  differs from the baseline capacitance C b  by more than the threshold difference C t  and responsively determine a depth  171  of the anomaly  146  beneath the first electrode  102  and the second electrode  104  (e.g., the electrode set  119 A) based on the first capacitance  108 , the second capacitance  161 , and the third capacitance  163 . Additionally, the user interface  230  provides an indication (e.g., a numeric display) of the depth  171  of the anomaly  146  beneath the first electrode  102  and the second electrode  104 . In some examples, crosstalk phenomena exist in which the electrode set  119 A, the electrode set  119 B, and the electrode set  119 C could all detect the anomaly  146 , with the respective deviation in capacitance from the baseline capacitance C b  being inversely proportional to the distance of the electrode set from the anomaly  146 . In this way, triangulation techniques could be applied to the first capacitance  108 , the second capacitance  161 , and the third capacitance  163  to determine the depth  171  and/or a size  173  (e.g., a diameter) of the anomaly  146 . Accordingly, the user interface  230  provides an indication (e.g., a numeric display) of the size  173  and/or the depth  171 . 
       FIGS. 19-26  are block diagrams of methods  200 ,  201 ,  203 ,  205 ,  300 ,  301 ,  303 , and  305  for operating a sensing instrument. The methods  200 ,  201 ,  203 ,  205 ,  300 ,  301 ,  303 , and  305  present examples of methods that could be used with the sensing instrument  100  and the material  110  as shown in  FIGS. 1-18 . As shown in  FIGS. 19-26 , the methods  200 ,  201 ,  203 ,  205 ,  300 ,  301 ,  303 , and  305  include one or more operations, functions, or actions as illustrated by blocks  202 ,  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  302 ,  304 ,  306 ,  308 ,  310 ,  312 ,  314 ,  316 ,  318 ,  320 ,  322 ,  324 ,  326 ,  328 ,  330 ,  332 ,  334 , and  336 . Although the blocks are illustrated in a sequential order, these blocks can also be performed in parallel, and/or in a different order than those described herein. Also, the various blocks can be combined into fewer blocks, divided into additional blocks, and/or removed based upon the desired implementation. 
       FIG. 19  is a block diagram of the method  200 . 
     At block  202 , the method  200  includes applying the first voltage  112  between the first electrode  102  and the second electrode  104  while the first electrode  102  and the second electrode  104  are positioned adjacent to the material  110 . In this context, the second electrode  104  surrounds the first electrode  102 . 
     At block  204 , the method  200  includes sensing the first capacitance  108  of the material  110  based on the response V 1  of the material  110  to the first voltage  112 . 
       FIG. 20  is a block diagram of the method  201 . 
     At block  206 , the method  201  includes applying the second voltage  135  between the first electrode  102  and the third electrode  114  concurrently with applying the first voltage  112 . The second voltage  135  is applied while the third electrode  114  is positioned adjacent to the material  110 . The third electrode  114  surrounds the second electrode  104 . 
       FIG. 21  is a block diagram of the method  203 . 
     At block  208 , the method  203  includes determining that the first capacitance  108  of the material  110  differs from the baseline capacitance C b  by more than the threshold difference C t . 
     At block  210 , the method  203  includes responsive to the determining, providing the indication  144  via the user interface  230  that the anomaly  146  exists beneath the first electrode  102  and the second electrode  104 . 
       FIG. 22  is a block diagram of the method  205 . 
     At block  212 , the method  205  includes sensing the first capacitance  108  of the material  110  while moving the first electrode  102  and the second electrode  104  over the material  110  to be adjacent to the testing region  148  of the material  110 . 
     At block  214 , the method  205  includes low-pass filtering the first capacitance  108  sensed while moving the first electrode  102  and the second electrode  104 . 
       FIG. 23  is a block diagram of the method  300 . 
     At block  302 , the method  300  includes applying the first voltage  112  between the first electrode  102  and the second electrode  104  while the first electrode  102  and the second electrode  104  are positioned adjacent to the first region  150  of the material  110 . In this context, the second electrode  104  surrounds the first electrode  102 . 
     At block  304 , the method  300  includes sensing the first capacitance  108  of the material  110  based on the first response V 1  of the material  110  to the first voltage  112 . 
     At block  306 , the method  300  includes applying the second voltage  136  between the third electrode  128  and the fourth electrode  130  while the third electrode  128  and the fourth electrode  130  are positioned adjacent to the second region  152  of the material  110 . In this context, the fourth electrode  130  surrounds the third electrode  128 . 
     At block  308 , the method  300  includes sensing the second capacitance  161  of the material  110  based on the second response V 2  of the material  110  to the second voltage  136 . 
     At block  310 , the method  300  includes applying the third voltage  138  between the fifth electrode  132  and the sixth electrode  134  while the fifth electrode  132  and the sixth electrode  134  are positioned adjacent to the third region  154  of the material  110 . In this context, the sixth electrode  134  surrounds the fifth electrode  132 . 
     At block  312 , the method  300  includes sensing the third capacitance  163  of the material  110  based on the third response V 3  of the material  110  to the third voltage  138 . 
       FIG. 24  is a block diagram of the method  301 . 
     At block  314 , the method  301  includes determining that the first capacitance  108  of the material  110  differs from the baseline capacitance C b  by more than the threshold difference C t . 
     At block  316 , the method  301  includes, responsive to the determining, determining the depth  171  of the anomaly  146  beneath the first electrode  102  and the second electrode  104  based on the first capacitance  108 , the second capacitance  161 , and the third capacitance  163 . 
     At block  318 , the method  301  includes providing the indication via the user interface  230  of the depth  171  of the anomaly  146  beneath the first electrode  102  and the second electrode  104 . 
       FIG. 25  is a block diagram of the method  303 . 
     At block  320 , the method  303  includes determining that the first capacitance  108  of the material  110  differs from the baseline capacitance C b  by more than the threshold difference C t . 
     At block  322 , the method  303  includes responsive to the determining, determining the size  173  of the anomaly  146  beneath the first electrode  102  and the second electrode  104  based on the first capacitance  108 , the second capacitance  161 , and the third capacitance  163 . 
     At block  324 , the method  303  includes providing the indication via the user interface  230  of the size  173  of the anomaly  146  beneath the first electrode  102  and the second electrode  104 . 
       FIG. 26  is a block diagram of the method  305 . 
     At block  326 , the method  305  includes applying the fourth voltage  512  between the first electrode  102  and the second electrode  104  while the first electrode  102  and the second electrode  104  are positioned adjacent to the fourth region  190  of the material  110 . 
     At block  328 , the method  305  includes sensing the fourth capacitance  508  of the material  110  based on the fourth response V 4  of the material  110  to the fourth voltage  512 . 
     At block  330 , the method  305  includes applying the fifth voltage  536  between the third electrode  128  and the fourth electrode  130  while the third electrode  128  and the fourth electrode  130  are positioned adjacent to the fifth region  192  of the material  110 . 
     At block  332 , the method  305  includes sensing the fifth capacitance  561  of the material  110  based on the fifth response V 5  of the material  110  to the fifth voltage  536 . 
     At block  334 , the method  305  includes applying the sixth voltage  538  between the fifth electrode  132  and the sixth electrode  134  while the fifth electrode  132  and the sixth electrode  134  are positioned adjacent to the sixth region  194  of the material  110 . 
     At block  336 , the method  305  includes sensing the sixth capacitance  563  of the material  110  based on the sixth response V 6  of the material  110  to the sixth voltage  538 . 
     Further, the disclosure comprises examples according to the following clauses: 
     Clause 1: A sensing instrument comprising: a first electrode; a second electrode that surrounds the first electrode; and a sensing module configured to sense a capacitance of a material by applying a voltage between the first electrode and the second electrode while the first electrode and the second electrode are adjacent to the material. 
     Clause 2: The sensing instrument of Clause 1, wherein the first electrode is coplanar with the second electrode. 
     Clause 3: The sensing instrument of any of Clauses 1 or 2, wherein the first electrode and the second electrode together have reflectional symmetry and rotational symmetry. 
     Clause 4: The sensing instrument of any of Clauses 1-3, further comprising a third electrode that surrounds the first electrode and the second electrode. 
     Clause 5: The sensing instrument of any of Clauses 1-4, further comprising a circuit board, the first electrode and the second electrode being positioned on a first side of the circuit board, the circuit board comprising a metal shielding layer on a second side of the circuit board that is opposite the first side. 
     Clause 6: The sensing instrument of any of Clauses 1-5, further comprising a spacer configured to maintain a minimum distance between (i) the first electrode or the second electrode and (ii) the material. 
     Clause 7: The sensing instrument of any of Clauses 1-6, further comprising: a third electrode; a fourth electrode that surrounds the third electrode; a fifth electrode; and a sixth electrode that surrounds the fifth electrode, wherein the first electrode, the third electrode, and the fifth electrode are collinear, and wherein the sensing module is further configured to: apply a second voltage between the third electrode and the fourth electrode while the third electrode and the fourth electrode are adjacent to the material, and apply a third voltage between the fifth electrode and the sixth electrode while the fifth electrode and the sixth electrode are adjacent to the material. 
     Clause 8: The sensing instrument of any of Clauses 1-6, further comprising: a third electrode; a fourth electrode that surrounds the third electrode; a fifth electrode; and a sixth electrode that surrounds the fifth electrode, wherein the first electrode, the third electrode, and the fifth electrode are not collinear, and wherein the sensing module is further configured to: apply a second voltage between the third electrode and the fourth electrode while the third electrode and the fourth electrode are adjacent to the material, and apply a third voltage between the fifth electrode and the sixth electrode while the fifth electrode and the sixth electrode are adjacent to the material. 
     Clause 9: A method of operating a sensing instrument, the method comprising: applying a voltage between a first electrode and a second electrode while the first electrode and the second electrode are positioned adjacent to a material, wherein the second electrode surrounds the first electrode; and sensing a capacitance of the material based on a response of the material to the voltage. 
     Clause 10: The method of Clause 9 wherein applying the voltage comprises applying the voltage while the first electrode and the second electrode are not in contact with the material. 
     Clause 11: The method of any of Clauses 9-10, wherein the voltage is an alternating current (AC) voltage. 
     Clause 12: The method of any of Clauses 9-11, wherein the voltage is a first voltage, the method further comprising applying a second voltage between the first electrode and a third electrode concurrently with applying the first voltage, wherein the second voltage is applied while the third electrode is positioned adjacent to the material, and wherein the third electrode surrounds the second electrode. 
     Clause 13: The method of Clause 12, wherein applying the second voltage comprises applying the second voltage to shape an electric field formed between the first electrode and the second electrode. 
     Clause 14: The method of any of Clauses 9-13, further comprising: determining that the capacitance of the material differs from a baseline capacitance by more than a threshold difference; and responsive to the determining, providing an indication via a user interface that an anomaly exists beneath the first electrode and the second electrode. 
     Clause 15: The method of any of Clauses 9-14, further comprising: sensing the capacitance of the material while moving the first electrode and the second electrode over the material to be adjacent to a second region of the material; and low-pass filtering the capacitance sensed while moving the first electrode and the second electrode. 
     Clause 16: A method of operating a sensing instrument, the method comprising: applying a first voltage between a first electrode and a second electrode while the first electrode and the second electrode are positioned adjacent to a first region of a material, wherein the second electrode surrounds the first electrode; sensing a first capacitance of the material based on a first response of the material to the first voltage; applying a second voltage between a third electrode and a fourth electrode while the third electrode and the fourth electrode are positioned adjacent to a second region of the material, wherein the fourth electrode surrounds the third electrode; sensing a second capacitance of the material based on a second response of the material to the second voltage; applying a third voltage between a fifth electrode and a sixth electrode while the fifth electrode and the sixth electrode are positioned adjacent to a third region of the material, wherein the sixth electrode surrounds the fifth electrode; and sensing a third capacitance of the material based on a third response of the material to the third voltage. 
     Clause 17: The method of Clause 16, further comprising: determining that the first capacitance of the material differs from a baseline capacitance by more than a threshold difference; responsive to the determining, determining a depth of an anomaly beneath the first electrode and the second electrode based on the first capacitance, the second capacitance, and the third capacitance; and providing an indication via a user interface of the depth of the anomaly beneath the first electrode and the second electrode. 
     Clause 18: The method of Clause 16, further comprising: determining that the first capacitance of the material differs from a baseline capacitance by more than a threshold difference; responsive to the determining, determining a size of an anomaly beneath the first electrode and the second electrode based on the first capacitance, the second capacitance, and the third capacitance; and providing an indication via a user interface of the size of the anomaly beneath the first electrode and the second electrode. 
     Clause 19: The method of any of Clauses 16-18, wherein the first electrode, the third electrode, and the fifth electrode are collinear, the method further comprising: applying a fourth voltage between the first electrode and the second electrode while the first electrode and the second electrode are positioned adjacent to a fourth region of the material; sensing a fourth capacitance of the material based on a fourth response of the material to the fourth voltage; applying a fifth voltage between the third electrode and the fourth electrode while the third electrode and the fourth electrode are positioned adjacent to a fifth region of the material; sensing a fifth capacitance of the material based on a fifth response of the material to the fifth voltage; applying a sixth voltage between the fifth electrode and the sixth electrode while the fifth electrode and the sixth electrode are positioned adjacent to a sixth region of the material; and sensing a sixth capacitance of the material based on a sixth response of the material to the sixth voltage. 
     Clause 20: The method of any of Clauses 16-18, wherein the first electrode, the third electrode, and the fifth electrode are not collinear. 
     The description of the different advantageous arrangements has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the examples in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different advantageous examples may describe different advantages as compared to other advantageous examples. The example or examples selected are chosen and described in order to explain the principles of the examples, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various examples with various modifications as are suited to the particular use contemplated.