Patent Publication Number: US-2023136092-A1

Title: Agricultural machines comprising capacitive sensors, and related methods and apparatus

Description:
FIELD 
     Embodiments of the present disclosure relate generally to sensors and methods of measuring material properties, such as of harvested crops. In particular, embodiments relate to methods and apparatus for generating electric fields that interact with materials. 
     BACKGROUND 
     When harvesting crops, information about the properties of the crop material (e.g., mass flow, moisture content, crop density, etc.) can be used to make decisions about how and when to operate machinery for improved yield. To increase the speed and efficiency of machines, it would be beneficial to have sensors that can quickly detect properties of crop material without interfering with operation of the machines. 
     Such sensors could also be used in other situations in which a nondestructive test (and potentially a non-contact test) is desirable, such as in detecting material in packages, in buildings (e.g., within wall structures), in manufacturing processes, mining, etc. 
     BRIEF SUMMARY 
     In some embodiments, an agricultural machine has a capacitive sensor that includes a transmitter assembly having a signal driver, at least one guard driver, and at least one sensing circuit configured to detect an output of the signal driver. At least one sensing electrode is powered by the signal driver. At least one guard electrode is powered by the at least one guard driver. The at least one guard electrode is oriented such that a first electric field emanating from the at least one sensing electrode is shaped at least in part by a second electric field emanating from the at least one guard electrode. 
     A method includes broadcasting a first electric field from a sensing electrode into a volume containing crop material, broadcasting a second electric field from at least one guard electrode adjacent the sensing electrode, measuring an attribute related to the first electric field, and correlating the measured attribute related to the first electric field to a property of the crop material in the volume. At least some field lines of the first electric field emanate from the sensing electrode through the volume. The presence of the second electric field changes a shape of the field lines of the first electric field. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming what are regarded as embodiments of the present disclosure, various features and advantages of embodiments of the disclosure may be more readily ascertained from the following description of example embodiments when read in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a simplified schematic view illustrating a capacitive sensor; 
         FIG.  2    is a simplified schematic view illustrating another capacitive sensor; 
         FIG.  3    is a simplified schematic view illustrating yet another a capacitive sensor; 
         FIG.  4    is another view of the sensor of  FIG.  3   ; 
         FIG.  5    is a simplified schematic view illustrating another capacitive sensor; 
         FIG.  6    is a simplified schematic view illustrating another capacitive sensor; 
         FIG.  7    is a simplified side view of a baling system, which may include the sensors shown in  FIGS.  1  through  6   ; 
         FIG.  8    is a simplified side view of a windrower, which may include the sensors shown in  FIGS.  1  through  6   ; 
         FIG.  9    is a simplified flow chart illustrating a method of measuring electric fields associated with crop material; and 
         FIG.  10    illustrates an example computer-readable storage medium comprising processor-executable instructions configured to embody one or more of the methods of measuring electric fields associated with crop material, such as the method illustrated in  FIG.  9   . 
     
    
    
     DETAILED DESCRIPTION 
     The illustrations presented herein are not actual views of any machine, sensor, or portion thereof, but are merely idealized representations that are employed to describe example embodiments of the present disclosure. Additionally, elements common between figures may retain the same numerical designation. 
     The following description provides specific details of embodiments of the present disclosure in order to provide a thorough description thereof. However, a person of ordinary skill in the art will understand that the embodiments of the disclosure may be practiced without employing many such specific details. Indeed, the embodiments of the disclosure may be practiced in conjunction with conventional techniques employed in the industry. In addition, the description provided below does not include all elements to form a complete structure or assembly. Only those process acts and structures necessary to understand the embodiments of the disclosure are described in detail below. Additional conventional acts and structures may be used. The drawings accompanying the application are for illustrative purposes only, and are thus not drawn to scale. 
     As used herein, the terms “comprising,” “including,” “containing,” “characterized by,” and grammatical equivalents thereof are inclusive or open-ended terms that do not exclude additional, unrecited elements or method steps, but also include the more restrictive terms “consisting of” and “consisting essentially of” and grammatical equivalents thereof. 
     As used herein, the term “may” with respect to a material, structure, feature, or method act indicates that such is contemplated for use in implementation of an embodiment of the disclosure, and such term is used in preference to the more restrictive term “is” so as to avoid any implication that other, compatible materials, structures, features, and methods usable in combination therewith should or must be excluded. 
     As used herein, the term “configured” refers to a size, shape, material composition, and arrangement of one or more of at least one structure and at least one apparatus facilitating operation of one or more of the structure and the apparatus in a predetermined way. 
     As used herein, the singular forms following “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     As used herein, spatially relative terms, such as “beneath,” “below,” “lower,” “bottom,” “above,” “upper,” “top,” “front,” “rear,” “left,” “right,” and the like, may be used for ease of description to describe one element&#39;s or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. Unless otherwise specified, the spatially relative terms are intended to encompass different orientations of the materials in addition to the orientation depicted in the figures. 
     As used herein, the term “substantially” in reference to a given parameter, property, or condition means and includes to a degree that one of ordinary skill in the art would understand that the given parameter, property, or condition is met with a degree of variance, such as within acceptable manufacturing tolerances. By way of example, depending on the particular parameter, property, or condition that is substantially met, the parameter, property, or condition may be at least 90.0% met, at least 95.0% met, at least 99.0% met, or even at least 99.9% met. 
     As used herein, the term “about” used in reference to a given parameter is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the given parameter). 
       FIG.  1    is a simplified diagram illustrating a capacitive sensor  100 . The sensor  100  includes a transmitter assembly  102 , at least one sensing electrode  104 , and at least one guard electrode  106 . The sensor  100  may also include one or more ground electrode(s)  108 . The sensor  100  may be considered a “guarded” capacitive sensor because it can generate two electric fields  124 ,  126 , which can interact with one another and shape one another (i.e., one can “guard” the other). In particular, the field  124  associated with the sensing electrode(s)  104  may be considered the sensing field, and the field  126  associated with the guard electrode(s)  106  may be considered the guard field. The guard field  126  may be used to shape the sensing field  124 , as described in further detail below. 
     The transmitter assembly  102  shown in  FIG.  1    includes a signal driver  110 , a guard driver  112 , and a sensing circuit  114 . A power source  138  may provide power to the components of the transmitter assembly  102 , and may be within or external to the transmitter assembly  102 . As illustrated, the signal driver  110  may be configured to provide a selected voltage to the sensing electrode  104  (which may be a constant or time-variable voltage). The sensing circuit  114  may measure the output of the signal driver  110 , such as the current or power levels required to provide the selected voltage. The guard driver  112  may be configured to provide a selected voltage to the guard electrode  106 , and may optionally receive its power from the power source  138  via the signal driver  110 . In other embodiments, the guard driver  112  receives its power directly from the power source  138 . 
     The sensing electrode  104  may have a major front surface  116  and a major rear surface  118  on an opposite side of the sensing electrode  104 , each of which may be generally planar. In other embodiments, the major front surface  116  and the major rear surface  118  may be curved or of another shape. For example, if the sensor  100  is designed to measure material in a tube, the major front surface  116  and the major rear surface  118  may have curvature matching the curvature of the tube. The major rear surface  118  may be generally aligned with the major front surface  116 . For example, if both are generally planar, the major front surface  116  may be parallel to the major rear surface  118  and separated by a distance smaller than either dimension (e.g., length or width) of the major front surface  116 . The sensing electrode  104  is powered by the signal driver  110  of the transmitter assembly  102 . 
     The guard electrode  106  may have similar geometry, with a major front surface  120  and a major rear surface  122 . The guard electrode  106  is powered by the guard driver  112  of the transmitter assembly  102 . The major front surface  120  of the guard electrode  106  may be located adjacent the major rear surface  118  of the sensing electrode  104 , such as separated by a distance smaller than the shortest of the lateral length or width of the sensing electrode  104 . 
       FIG.  1    depicts a few field lines of the electric field  124  of the sensing electrode  104  and the electric field  126  of the guard electrode  106 . The major front surface  120  of the guard electrode  106  may be oriented such that the field  124  of the sensing electrode  104  is shaped at least in part by the field  126  of the guard electrode  106 . As shown in  FIG.  1   , the electrodes may be positioned such that the distance from the major rear surface  118  of the sensing electrode  104  to the major front surface  120  of the guard electrode  106  is smaller than the distance from the major rear surface  118  of the sensing electrode  104  to the ground electrode  108  (or to any other electrode, if any, or any other object). The shortest path from any point on the major rear surface  118  of the sensing electrode  104  outward (i.e., in a direction generally downward in the view of  FIG.  1   ) may intersect the major front surface  120  of the guard electrode  106  before reaching any other object. In this way, the guard electrode  106  may prevent other objects from interfering with the portion of the field  124  emanating from the major rear surface  118  of the sensing electrode  104 . Thus, the guard electrode  106  may “guard” the major rear surface  118  of the sensing electrode  104  from interacting with other electrodes, and may cause the sensing electrode  104  to function similar to a theoretical one-sided electrode (i.e., with no field lines emanating from the major rear surface  118  of the sensing electrode  104  past the guard electrode  106 ). In such an arrangement, the field lines of the field  124  emanating from the major rear surface  118  of the sensing electrode  104  are spaced more densely than field lines emanating from the major front surface  116 . Thus, the field lines of the field  124  of the sensing electrode  104  do not pass or intersect the guard electrode  106 . 
     The ground electrode  108 , if present, may be separated from and coplanar with the sensing electrode  104 . In such embodiments, some field lines of the field  124  of the sensing electrode  104  may have an arcuate shape extending outward from the major front surface  116  of the sensing electrode  104  to the ground electrode  108 . Some field lines of the field  126  of the guard electrode  106  may have an arcuate shape extending outward from the major rear surface  122  of the guard electrode  106  to the ground electrode  108  (which may be the same ground electrode  108  or a different ground electrode  108 ). The ground electrode(s)  108  may laterally surround the sensing electrode  104  and/or the guard electrode  106 . In some embodiments, the ground electrode  108  may include a shield  128  protecting the transmitter assembly  102 , such that the fields  124 ,  126  do not interfere with the operation of the transmitter assembly  102 . Field lines emanating from the sensing electrode  104  and the guard electrode  106  may intersect the ground electrode  108 . 
     In some embodiments, a cable  130  may connect the transmitter assembly  102  to the electrodes  104 ,  106 ,  108 . The cable  130  may be a coaxial cable having two or more conductors sharing a common axis. For example, the cable  130  may have a first conductor, shown as an inner core conductor (e.g., a wire) of a triaxial cable, connecting the sensing circuit  114  to the sensing electrode  104 . The cable  130  may have a second conductor, shown as an intermediate cylindrical conductor of a triaxial cable, connecting the guard driver  112  to the guard electrode  106 . The cable  130  may have a third conductor, shown as an outer cylindrical conductor of a triaxial cable, connecting the ground electrode  108  to a physical ground. In some embodiments, the ground electrode  108  may be omitted, and thus, the cable  130  may be a biaxial cable, having only two conductors. 
     The transmitter assembly  102  is configured to provide a first signal to the sensing electrode  104  and a second signal to the guard electrode  106 . For example, and as discussed above, the transmitter assembly  102  may send preselected voltage signals to the electrodes  104 ,  106 . In some embodiments, the voltage provided to each electrode  104 ,  106  may be identical in magnitude but electrically isolated. Isolated outputs may enable the transmitter assembly  102  to distinguish material sensed in the volume encompassed by field  124  from  126 , and that the interference between them may be insignificant. That is, the magnitude of the difference between the two fields  124 ,  126  as they extend outward, and/or the field established between the major rear surface  118  of the sensing electrode  104  and the major front surface  120  of the guard electrode  106  may be significantly smaller (e.g., at least one order of magnitude, at least two orders of magnitude, etc.) than the magnitude of the fields  124 ,  126  themselves. 
     The transmitter assembly  102  may provide the selected signals to the electrodes  104 ,  106  using the signal driver  110  and the guard driver  112 . The power source  138  may provide power to the signal driver  110  and the guard driver  112 . 
     In some embodiments, the transmitter assembly  102  may be configured to provide a first voltage to the sensing electrode  104  and a second, different voltage to the guard electrode  106 . The second voltage may be offset from the first voltage by a preselected amount. These different voltages may be useful for active sensing of material, using changing field parameters (e.g., detection area size or shape, direction of view, calibration, etc.). 
     The sensing circuit  114  may measure the output of the signal driver  110 , such as the current or power levels required to provide the selected voltage. The guard driver  112  may be configured to provide a selected voltage to the guard electrode  106 , and may optionally receive its power from the power source  138  via the signal driver  110 . Though the transmitter assembly  102  is described as providing a known voltage and measuring known power or current, other attributes of the field may be measured, such as current, power, voltage, reactance, impedance, resonance, capacitance, frequency, permittivity, time, etc. 
     The sensing field  124  may have a response curve, meaning that its attributes vary in a particular way in response to different conditions. For example, the sensing field  124  may have a field strength that decreases in proportion to 1/r 2  or 1/r 3 , where r is the distance from the major front surface  116  of the sensing electrode  104 . If the field  124  is formed by electromagnetic radiation having a frequency that excites water molecules, moisture in the field  124  can affect the field lines of the field  124 . The material in the field  124  may cause a change in the electrical load provided by the signal driver  110  to generate the field  124 , and may be measured by the sensing circuit  114 . The use of electromagnetic sensors for characterizing crop material is described in more detail in U.S. Provisional Patent Application ______, “Methods of Measuring Crop Material,” filed on the same date as this application in the name of Honeyman, et al. 
       FIG.  2    is a simplified diagram illustrating another capacitive sensor  200 . The sensor  200  includes a transmitter assembly  202 , at least one sensing electrode  104 , and at least one guard electrode  206 . The sensor  200  is depicted as lacking a ground electrode, and with the guard electrode  206  also serving as the shield  128  to protect the transmitter assembly  202  from interference by the fields  224 ,  226  emanating from the electrodes  104 ,  206 . As depicted, the fields  224 ,  226  may extend generally outward to another physical ground. The electrodes  104 ,  206  may be arranged such that the guard field  226  shapes the sensing field  224 . In the embodiment shown, the field lines of the sensing field  224  extend generally upward and outward. 
       FIG.  3    is a simplified diagram illustrating another capacitive sensor  300 . The sensor  300  includes a transmitter assembly  302 , a sensing electrode  104 , a guard electrode  306 , switchable electrodes  307   a - 307   e , and a ground electrode  108  (which may operate as a shield  128  to protect the transmitter assembly  302  from interference, either alone or in combination with the guard electrode  306 ). The major front surfaces of the electrodes  104 ,  306 ,  307   a - 307   e  may be generally coplanar. In embodiments in which the major front surfaces are curved or another shape, the curvature of the major front surfaces of the electrodes  104 ,  306 ,  307   a - 307   e  may be generally continuous. 
     The transmitter assembly  302  may include a signal driver  110 , a guard driver  112 , a sensing circuit  114 , and a power source  138 , as discussed above. Furthermore, the transmitter assembly  302  may include a multiplexor  340  and a controller  342 . The multiplexor  340  may be configured to selectively provide power from the guard driver  112  to individual switchable electrodes  307   a - 307   e . The controller  342  can drive the multiplexor  340  to change which of the switchable electrodes  307   a - 307   e  are powered and which are grounded. Thus, the switchable electrodes  307   a - 307   e  may, when so powered, operate as guard electrodes, similar to the guard electrode  306 . The multiplexor  340  may also ground one of the switchable electrodes  307   a - 307   e . As depicted in  FIG.  3   , the electrode  307   b  is grounded by the multiplexor  340 , while the electrodes  307   a ,  307   c ,  307   d , and  307   e  are powered by the guard driver  112 . Thus, field lines of the guard field  326  connect the guard electrodes  306 ,  307   a ,  307   c ,  307   d , and  307   e  to the grounded switchable electrode  307   b  and/or to the ground electrode  108 . The field lines of the sensing field  324  connect the sensing electrode  104  to the grounded switchable electrode  307   b . The approximate shape of the sensing field  324  is shaded in  FIG.  3   . 
       FIG.  4    is a simplified diagram illustrating how the shape of the fields changes when the multiplexor  340  changes which electrodes are powered. In particular, switchable electrode  307   c  is depicted as grounded in  FIG.  4   , and the guard field  426  has field lines connecting the electrodes  306 ,  307   a ,  307   b ,  307   d , and  307   e  to the grounded switchable electrode  307   c  and/or to the ground electrode  108 . The field lines of the sensing field  424  connect the sensing electrode  104  to the grounded switchable electrode  307   c . The approximate shape of the sensing field  424  is shaded in  FIG.  4   . By changing which of the switchable electrodes  307   a - 307   e  are powered and which are grounded, the shape of the sensing field  324 ,  424  can be changed without changing the physical location of the electrodes  104 ,  108 ,  306 ,  307   a - 307   e . The multiplexor  340  may likewise ground any of the switchable electrodes  307   a - 307   e  to cause different sensing fields. Thus, the multiplexor  340  may enable rapid switching between sensing fields of different size and/or shape without physical movement of the sensor  300  or components thereof. 
       FIG.  5    is a simplified diagram illustrating another capacitive sensor  500 . The sensor  500  may be generally configured similar to the sensor  300  shown in  FIGS.  3  and  4   , including a transmitter assembly  502 , a sensing electrode  104 , a guard electrode  306 , switchable electrodes  307   a - 307   e , and a ground electrode  108  (which also operates as a shield  128  to protect the transmitter assembly  302  from interference). The transmitter assembly  502  differs from the transmitter assembly  302  shown in  FIG.  3    in that it includes switched guard drivers  513   a - 513   e , instead of the multiplexor  340 . One switched guard driver  513   a - 513   e  is configured to power each of the switchable electrodes  307   a - 307   e . The switched guard drivers  513   a - 513   e  may be independently driven by the controller  342 , such that the transmitter assembly  502  can generate different sensing and guard fields in a similar manner as the sensor  300 . 
     In some embodiments, different sensing electrodes may form different sensing fields concurrently. For example,  FIG.  6    is a simplified diagram illustrating another capacitive sensor  600 . The sensor  600  includes a transmitter assembly  602 , sensing electrodes  604   a - 604   e , a guard electrode  306 , and a ground electrode  108  (which also operates as a shield  128  to protect the transmitter assembly  602  from interference). 
     The transmitter assembly  602  includes a signal driver  110  configured to provide selected voltage(s) to the guard electrode  306  and the sensing electrodes  604   a - 604   e . Sensing circuits  615   a - 615   e  may selectivity detect the output of the signal driver  110  to each corresponding sensing electrode  604   a - 604   e . An output driver  644  may control voltages applied to each of the sensing electrodes  604   a - 604   e . Sensing fields  624  may form between each sensing electrode  604   a - 604   e  and the ground electrode  108 . Each sensing electrode  604   a - 604   e  may generate a separate sensing field, and the size and shape of each is dependent on the distance from each sensing electrode  604   a - 604   e  to the ground electrode  108 , the position of the other sensing electrodes  604   a - 604   e , the power levels of each sensing electrode  604   a - 604   e , the size and shape of each sensing electrode  604   a - 604   e , etc. The different sensing fields  624  may shape one another, and may together be used to measure properties of material in different volumes or in different ways. 
     Each of the sensors  300 ,  500 ,  600  shown in  FIGS.  3  through  6    include a guard electrode  306  depicted on the left of the sensing electrode(s)  104  or  604   a - 604   e . However, in some embodiments, the guard electrode  306  may be omitted and replaced by additional switchable electrodes or sensing electrodes. In such embodiments, the additional switchable electrodes or sensing electrodes may serve to shape the sensing fields and/or limit the influence of external electric fields on the sensing fields. Furthermore, the sensors  100 ,  200 ,  300 ,  500 ,  600  shown in  FIG.  1  through  6    are depicted as unitary structures (either combined with or separate from the transmitter assemblies), but the various electrodes may also be configured to be separately mounted on a body or on separate bodies in such a manner that fields generated affect one another. 
     The sensors  100 ,  200 ,  300 ,  500 ,  600  described herein may carried by an agricultural vehicle frame and may be used to measure crop material in crop-harvesting operations, such as in combines, windrowers, balers, etc. The sensors  100 ,  200 ,  300 ,  500 ,  600  and methods herein may also be used to measure properties of any other type of material, and may be used in various industries, such as mining, chemical processing, food processing and packaging, shipping, security (e.g., nondestructively detecting properties of unopened parcels), construction (e.g., detecting materials inside walls), manufacturing (e.g., nondestructive parts inspection), etc. 
       FIG.  7    is a simplified side view of a baling system  700  including a tractor  702  towing a baler  704 , each of which include vehicle frames. The baling system  700  is operable to receive loose crop material  704 , form it into individual charges, and compress the charges to form a bale  706 . The baler  702  may include a stuffing component  708 , a forming chamber  710 , and a plunger  712 . The stuffing component  708  picks up the loose crop material  704 , and transfers it to the forming chamber  710 . The plunger  712  compresses the loose crop material  704  to form the bale  706  or a portion thereof. Baling systems are described in more detail in International Patent Application Publication WO 2019/123039, “Baler with NIR Sensor in Plunger Face,” published 27 Jun. 2019. As depicted in  FIG.  7   , the baler  704  may include one or more sensors  714 ,  716 ,  718 . For example, the sensor  714  is depicted adjacent to the stuffing component  708  to detect properties of the loose crop material  704  entering the baler  704 . The sensor  716  is depicted on or in the plunger  712  to detect properties of the crop material before or during compression. The sensor  718  is depicted adjacent the bale  706  to detect properties of the crop material in the bale  706  as the bale  706  is ejected from the baler  704 . The sensors  714 ,  716 ,  718  may be a sensor  100 ,  200 ,  300 ,  500 ,  600  as shown in  FIGS.  1  through  6    and described above. Additional sensors may be in other locations within the baling system  700 , such as carried by the tractor  702 . Multiple sensors may be used to characterize whether operating parameters of the baling system  700  are effective, and may enable a control system to adjust the operating parameters (e.g., ground speed, compaction force, etc.) to improve the properties of the bale  706 . For example, the sensor(s)  714 ,  716 ,  718  may be used to measure the moisture content and/or density of crop material in the bales  706 . 
       FIG.  8    is a simplified side view of a self-propelled windrower  810 . In some embodiments, pull-type or other types of harvesting machines may be used. The windrower  810  broadly includes a self-propelled tractor  812  having a vehicle frame and a harvesting header  814  attached to and carried by the front of the tractor  812 . An operator drives the windrower  810  from a cab  816 , which includes an operator station having a tractor seat and one or more user interfaces (e.g., FNR joystick, display monitor, switches, buttons, etc.) that enable the operator to control various functions of the tractor  812  and header  814 . In one embodiment, a controller  817  or computing system is disposed in the cab  816 , though in some embodiments, the controller  817  may be located elsewhere or include a distributed architecture having plural computing devices, coupled to one another in a network, throughout various locations within the tractor  812  (or in some embodiments, located in part externally and in remote communication with one or more local computing devices). 
     The header  814  includes a cutter  818 , a conditioning system, a swathboard  824 , and a forming shield assembly  822 . The cutter  818  is configured for severing standing crops as the windrower  810  moves through the field. The conditioning system, in the depicted embodiment, includes one or more pairs of conditioner rolls  820 . The forming shield assembly  822  may include a pair of rearwardly converging windrow forming shields located behind the conditioner rolls  820 . The swathboard  824  is located between the conditioner rolls  820  and the forming shield assembly  822 . In some embodiments, the conditioning system may be of a different design, such as a flail-type conditioning system. The swathboard  824  and/or the forming shield assembly  822  may be adjusted by one or more actuators  830 . 
     A sensor  826 , which may be any of the sensors  100 ,  200 ,  300 ,  500 ,  600  described above and shown in  FIGS.  1  through  6   , may be carried by the windrower  810  or the header  814  such that it can measure the crop material being cut by the header  814  and formed into a windrow. The measuring device  826  may communicate with the controller  817  such that the controller  817  can change operating parameters of the windrower  810  and/or the header  814  (e.g., a position of one or more of the actuators  830 ). In some embodiments, the measuring device  826  may report information to the operator, and the operator may make changes to the operating parameters of the windrower  810  and/or the header  814 . Changing operating parameters of a windrower  810  or header  814  based on information about the crop is described in more detail in U.S. Provisional Patent Application ______, “Agricultural Machines and Methods for Controlling Windrow Properties,” filed on the same date as this application in the name of Hamilton, et al. 
       FIG.  9    is a simplified flow chart illustrating a method  900  of measuring electric fields associated with crop material. In block  902 , a first electric field is broadcast from a sensing electrode into a volume containing crop material. At least some field lines of the first electric field emanate from the sensing electrode through the volume. The first electric field may be broadcast by applying a first voltage to the sensing electrode, which may be a constant voltage or a variable voltage. In block  904 , a second electric field is broadcast from at least one guard electrode adjacent the sensing electrode. The presence of the second electric field changes a shape of the field lines of the first electric field. The second electric field may be broadcast by applying a second voltage to the sensing electrode, which may be the same as or different from the first voltage. In some embodiments, the sensing electrode may be electrically isolated from the guard electrode. 
     In block  906 , an attribute of the first electric field is measured. For example, the attribute measured may be current, power, voltage, reactance, impedance, resonance, capacitance, frequency, permittivity, time, etc. In block  908 , the measured attribute is correlated to the first electric field to a property of the crop material in the volume. 
     In block  910 , an operating parameter of an agricultural machine is modified based on the property of crop material in the volume. 
     Still other embodiments involve a computer-readable storage medium (e.g., a non-transitory computer-readable storage medium) having processor-executable instructions configured to implement one or more of the techniques presented herein. An example computer-readable medium that may be devised is illustrated in  FIG.  10   , wherein an implementation  1000  includes a computer-readable storage medium  1002  (e.g., a flash drive, CD-R, DVD-R, application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), a platter of a hard disk drive, etc.), on which is computer-readable data  1004 . This computer-readable data  1004  in turn includes a set of processor-executable instructions  1006  configured to operate according to one or more of the principles set forth herein. In some embodiments, the processor-executable instructions  1006  may be configured to cause a computer associated with an agricultural machine, such as the baling system  700  ( FIG.  7   ) or the windrower  810  ( FIG.  8   ) to perform operations  1008  when executed via a processing unit, such as at least some of the example method  900  depicted in  FIG.  9   . In other embodiments, the processor-executable instructions  1006  may be configured to implement a system, such as at least some of the example baling system  700  ( FIG.  7   ) or windrower  810  ( FIG.  8   ). Many such computer-readable media may be devised by those of ordinary skill in the art that are configured to operate in accordance with one or more of the techniques presented herein. 
     Additional non limiting example embodiments of the disclosure are described below. 
     Embodiment 1: An agricultural machine comprising a vehicle frame and a capacitive sensor carried by the vehicle frame. The capacitive sensor comprises a transmitter assembly comprising a signal driver, at least one guard driver, and at least one sensing circuit configured to detect an output of the signal driver; at least one sensing electrode powered by the signal driver; and at least one guard electrode powered by the at least one guard driver. The at least one guard electrode is oriented such that a first electric field emanating from the at least one sensing electrode is shaped at least in part by a second electric field emanating from at least one the guard electrode. 
     Embodiment 2: The agricultural machine of Embodiment 1, wherein the sensor further comprises a cable comprising a first conductor connecting the at least one sensing circuit to the at least one sensing electrode and a second conductor connecting the at least one guard driver to the at least one guard electrode. 
     Embodiment 3: The agricultural machine of Embodiment 2, wherein the cable comprises a coaxial cable. 
     Embodiment 4: The agricultural machine of Embodiment 3, wherein the first conductor comprises an inner conductor of the coaxial cable and connects the sensing circuit to the at least one sensing electrode, and wherein the second conductor comprises an outer conductor of the coaxial cable and connects the guard driver to the at least one guard electrode. 
     Embodiment 5: The agricultural machine of Embodiment 1, further comprising at least one ground electrode. 
     Embodiment 6: The agricultural machine of Embodiment 5, wherein at least a portion of the at least one ground electrode is separated from and coplanar to the at least one sensing electrode. 
     Embodiment 7: The agricultural machine of Embodiment 5 or Embodiment 6, wherein the at least one ground electrode laterally surrounds the at least one sensing electrode. 
     Embodiment 8: The agricultural machine of any one of Embodiment 5 through Embodiment 7, wherein the at least one ground electrode shields the transmitter assembly. 
     Embodiment 9: The agricultural machine of any one of Embodiment 5 through Embodiment 8, wherein the sensor further comprises a cable comprising a first conductor connecting the at least one sensing circuit to the at least one sensing electrode, a second conductor connecting the at least one guard driver to the at least one guard electrode, and a third conductor connecting the at least one ground electrode to a ground of the transmitter assembly. 
     Embodiment 10: The agricultural machine of Embodiment 9, wherein the cable comprises a triaxial cable. 
     Embodiment 11: The agricultural machine of any one of Embodiment 5 through Embodiment 10, wherein field lines emanating from the at least one sensing electrode intersect the at least one ground electrode. 
     Embodiment 12: The agricultural machine of any one of Embodiment 1 through Embodiment 11, wherein field lines emanating from a major rear surface of the at least one sensing electrode are spaced more densely than field lines emanating from a major front surface of the at least one sensing electrode. 
     Embodiment 13: The agricultural machine of any one of Embodiment 1 through Embodiment 12, wherein the transmitter assembly is configured to provide a first voltage to the at least one sensing electrode and a second voltage to the at least one guard electrode, wherein the first voltage and the second voltage are identical in magnitude but electrically isolated. 
     Embodiment 14: The agricultural machine of any one of Embodiment 1 through Embodiment 12, wherein the transmitter assembly is configured to provide a first voltage to the at least one sensing electrode and a second voltage to the at least one guard electrode, wherein the second voltage is offset from the first voltage by a preselected amount. 
     Embodiment 15: The agricultural machine of any one of Embodiment 1 through Embodiment 14, wherein the at least one guard electrode shields the transmitter assembly. 
     Embodiment 16: The agricultural machine of any one of Embodiment 1 through Embodiment 15, wherein the signal driver and the at least one guard driver share a common power source. 
     Embodiment 17: The agricultural machine of any one of Embodiment 1 through Embodiment 16, wherein a major rear surface of the at least one sensing electrode is adjacent a major front surface of the at least one guard electrode. 
     Embodiment 18: The agricultural machine of Embodiment 17, wherein a shortest path from any point on the major rear surface of the at least one sensing electrode outward intersects the major front surface of the at least one guard electrode before reaching any other object. 
     Embodiment 19: The agricultural machine of any one of Embodiment 1 through Embodiment 16, wherein a major front surface of the at least one sensing electrode is coplanar with a major front surface of the at least one guard electrode. 
     Embodiment 20: The agricultural machine of any one of Embodiment 1 through Embodiment 19, wherein the at least one guard electrode comprises a plurality of guard electrodes. 
     Embodiment 21: The agricultural machine of Embodiment 20, wherein the transmitter further comprises a multiplexor configured to selectively provide power from the at least one guard driver to individual guard electrodes of the plurality. 
     Embodiment 22: The agricultural machine of Embodiment 21, wherein the sensor further comprises a controller configured to drive the multiplexor. 
     Embodiment 23: The agricultural machine of Embodiment 20, wherein the at least one guard driver comprises a plurality of switched guard drivers, and wherein each switched guard driver of the plurality is configured to selectively provide power to individual guard electrodes of the plurality. 
     Embodiment 24: The agricultural machine of Embodiment 23, wherein the transmitter assembly further comprises a controller configured to drive the plurality of switched guard drivers. 
     Embodiment 25: The agricultural machine of any one of Embodiment 1 through Embodiment 24, wherein the at least one sensing electrode comprises a plurality of sensing electrodes. 
     Embodiment 26: The agricultural machine of Embodiment 25, wherein the at least one sensing circuit comprises a plurality of sensing circuits, and wherein each sensing circuit of the plurality is configured to selectively detect power provided to individual sensing electrodes of the plurality by the signal driver. 
     Embodiment 27: The agricultural machine of Embodiment 25, wherein the transmitter assembly further comprises an output driver configured to control voltages applied to of each of the plurality of sensing electrodes. 
     Embodiment 28: The agricultural machine of any one of Embodiment 1 through Embodiment 27, wherein the vehicle frame comprises a baler. 
     Embodiment 29: The agricultural machine of Embodiment 28, wherein the sensor is arranged such that hay traveling through the baler passes adjacent the at least one sensing electrode and the at least one guard electrode. 
     Embodiment 30: The agricultural machine of any one of Embodiment 1 through Embodiment 27, wherein the vehicle frame comprises a windrower, wherein the sensor is arranged such that the sensing electrode and the at least one guard electrode pass crop material as the windrower travels through a field. 
     Embodiment 31: A method comprising broadcasting a first electric field from a sensing electrode into a volume containing crop material, broadcasting a second electric field from at least one guard electrode adjacent the sensing electrode, measuring an attribute related to the first electric field, and correlating the measured attribute related to the first electric field to a property of the crop material in the volume. At least some field lines of the first electric field emanate from the sensing electrode through the volume. The presence of the second electric field changes a shape of the field lines of the first electric field. 
     Embodiment 32: The method of Embodiment 31, where the measured attribute comprises an attribute selected from the group consisting of current, power, voltage, reactance, impedance, resonance, capacitance, frequency, permittivity, and time. 
     Embodiment 33: The method of Embodiment 31 or Embodiment 32, wherein broadcasting a first electric field comprises applying a first voltage to the sensing electrode, and wherein broadcasting a second electric field comprises applying the first voltage to the at least one guard electrode. 
     Embodiment 34: The method of Embodiment 33, further comprising electrically isolating the sensing electrode from the at least one guard electrode. 
     Embodiment 35: The method of any one of Embodiment 31 through Embodiment 34, further comprising modifying an operating parameter of an agricultural machine based on the property of crop material in the volume. 
     Embodiment 36: A non-transitory computer-readable storage medium, the computer-readable storage medium including instructions that when executed by a computer, cause the computer to perform the method of any one of Embodiment 31 through Embodiment 35. 
     Embodiment 37: A capacitive sensor, comprising a transmitter assembly comprising a signal driver, at least one guard driver, and at least one sensing circuit configured to detect an output of the signal driver; at least one sensing electrode powered by the signal driver; and at least one guard electrode powered by the at least one guard driver. The at least one guard electrode is oriented such that a first electric field emanating from the at least one sensing electrode is shaped at least in part by a second electric field emanating from at least one the guard electrode. 
     Embodiment 38: The sensor of Embodiment 37, further comprising a cable comprising a first conductor connecting the at least one sensing circuit to the at least one sensing electrode and a second conductor connecting the at least one guard driver to the at least one guard electrode. 
     Embodiment 39: The sensor of Embodiment 38, wherein the cable comprises a coaxial cable. 
     Embodiment 40: The sensor of Embodiment 39, wherein the first conductor comprises an inner conductor of the coaxial cable and connects the sensing circuit to the at least one sensing electrode, and wherein the second conductor comprises an outer conductor of the coaxial cable and connects the guard driver to the at least one guard electrode. 
     Embodiment 41: The sensor of Embodiment 37, further comprising at least one ground electrode. 
     Embodiment 42: The sensor of Embodiment 41, wherein at least a portion of the at least one ground electrode is separated from and coplanar to the at least one sensing electrode. 
     Embodiment 43: The sensor of Embodiment 41 or Embodiment 42, wherein the at least one ground electrode laterally surrounds the at least one sensing electrode. 
     Embodiment 44: The sensor of any one of Embodiment 41 through Embodiment 43, wherein the at least one ground electrode shields the transmitter assembly. 
     Embodiment 45: The sensor of any one of Embodiment 41 through Embodiment 44, further comprising a cable comprising a first conductor connecting the at least one sensing circuit to the at least one sensing electrode, a second conductor connecting the at least one guard driver to the at least one guard electrode, and a third conductor connecting the at least one ground electrode to a ground of the transmitter assembly. 
     Embodiment 46: The sensor of Embodiment 45, wherein the cable comprises a triaxial cable. 
     Embodiment 47: The sensor of any one of Embodiment 41 through Embodiment 46, wherein field lines emanating from the at least one sensing electrode intersect the at least one ground electrode. 
     Embodiment 48: The sensor of any one of Embodiment 37 through Embodiment 47, wherein field lines emanating from a major rear surface of the at least one sensing electrode are spaced more densely than field lines emanating from a major front surface of the at least one sensing electrode. 
     Embodiment 49: The sensor of any one of Embodiment 37 through Embodiment 48, wherein the transmitter assembly is configured to provide a first voltage to the at least one sensing electrode and a second voltage to the at least one guard electrode, wherein the first voltage and the second voltage are identical in magnitude but electrically isolated. 
     Embodiment 50: The sensor of any one of Embodiment 37 through Embodiment 48, wherein the transmitter assembly is configured to provide a first voltage to the at least one sensing electrode and a second voltage to the at least one guard electrode, wherein the second voltage is offset from the first voltage by a preselected amount. 
     Embodiment 51: The sensor of any one of Embodiment 37 through Embodiment 50, wherein the at least one guard electrode shields the transmitter assembly. 
     Embodiment 52: The sensor of any one of Embodiment 37 through Embodiment 51, wherein the signal driver and the at least one guard driver share a common power source. 
     Embodiment 53: The sensor of any one of Embodiment 37 through Embodiment 52, wherein a major rear surface of the at least one sensing electrode is adjacent a major front surface of the at least one guard electrode. 
     Embodiment 54: The sensor of Embodiment 53, wherein a shortest path from any point on the major rear surface of the at least one sensing electrode outward intersects the major front surface of the at least one guard electrode before reaching any other object. 
     Embodiment 55: The sensor of any one of Embodiment 37 through Embodiment 52, wherein a major front surface of the at least one sensing electrode is coplanar with a major front surface of the at least one guard electrode. 
     Embodiment 56: The sensor of any one of Embodiment 37 through Embodiment 55, wherein the at least one guard electrode comprises a plurality of guard electrodes. 
     Embodiment 57: The sensor of Embodiment 56, wherein the transmitter further comprises a multiplexor configured to selectively provide power from the at least one guard driver to individual guard electrodes of the plurality. 
     Embodiment 58: The sensor of Embodiment 57, further comprising a controller configured to drive the multiplexor. 
     Embodiment 59: The sensor of Embodiment 56, wherein the at least one guard driver comprises a plurality of switched guard drivers, and wherein each switched guard driver of the plurality is configured to selectively provide power to individual guard electrodes of the plurality. 
     Embodiment 60: The sensor of Embodiment 59, wherein the transmitter assembly further comprises a controller configured to drive the plurality of switched guard drivers. 
     Embodiment 61: The sensor of any one of Embodiment 37 through Embodiment 60, wherein the at least one sensing electrode comprises a plurality of sensing electrodes. 
     Embodiment 62: The sensor of Embodiment 61, wherein the at least one sensing circuit comprises a plurality of sensing circuits, and wherein each sensing circuit of the plurality is configured to selectively detect power provided to individual sensing electrodes of the plurality by the signal driver. 
     Embodiment 63: The sensor of Embodiment 61, wherein the transmitter assembly further comprises an output driver configured to control voltages applied to of each of the plurality of sensing electrodes. 
     Embodiment 64: A method comprising broadcasting a first electric field from a sensing electrode into a volume, broadcasting a second electric field from at least one guard electrode adjacent the sensing electrode, measuring an attribute related to the first electric field, and correlating the measured attribute related to the first electric field to a property of material in the volume. At least some field lines of the first electric field emanate from the sensing electrode through the volume. The presence of the second electric field changes a shape of the field lines of the first electric field. 
     Embodiment 65: The method of Embodiment 64, where the measured attribute comprises an attribute selected from the group consisting of current, power, voltage, reactance, impedance, resonance, capacitance, frequency, permittivity, and time. 
     Embodiment 66: The method of Embodiment 64 or Embodiment 65, wherein broadcasting a first electric field comprises applying a first voltage to the sensing electrode, and wherein broadcasting a second electric field comprises applying the first voltage to the at least one guard electrode. 
     Embodiment 67: The method of Embodiment 66, further comprising electrically isolating the sensing electrode from the at least one guard electrode. 
     Embodiment 68: The method of any one of Embodiment 64 through Embodiment 67, further comprising modifying an operating parameter of a machine based on the property of material in the volume. 
     Embodiment 69: A non-transitory computer-readable storage medium, the computer-readable storage medium including instructions that when executed by a computer, cause the computer to perform the method of any one of Embodiment 64 through Embodiment 68. 
     All references cited herein are incorporated herein in their entireties. If there is a conflict between definitions herein and in an incorporated reference, the definition herein shall control. 
     While the present disclosure has been described herein with respect to certain illustrated embodiments, those of ordinary skill in the art will recognize and appreciate that it is not so limited. Rather, many additions, deletions, and modifications to the illustrated embodiments may be made without departing from the scope of the invention as hereinafter claimed, including legal equivalents thereof. In addition, features from one embodiment may be combined with features of another embodiment while still being encompassed within the scope of the invention as contemplated by the inventors. Further, embodiments of the disclosure have utility with different and various crop-harvesting machine types and configurations.