Patent Publication Number: US-6669557-B2

Title: Apparatus for measuring parameters of material

Description:
REFERENCE TO RELATED APPLICATIONS 
     This is a Divisional of application Ser. No. 09/825,498, filed Apr. 3, 2001 and issued on Dec. 3, 2002 as U.S. Pat. No. 6,489,784 which is a Divisional of application Ser. No. 09/027,179, filed Feb. 23, 1998 and issued on Jun. 5, 2001 as U.S. Pat No. 6,242,927 which is continuation-in-part of application Ser. No. 08/835,610, filed on Apr. 9, 1997 and issued on Sep. 19, 2000 as U.S. Pat. No. 6,121,782. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of measuring parameters of material. More particularly, the invention relates to a method and an apparatus for measuring parameters of a material by driving a sensing element with multiple simultaneous frequency signals, generating signals responsive to the frequency response of the material at each frequency, and processing the generated signals to determine the parameters of the material. 
     BACKGROUND OF THE INVENTION 
     Various mobile and stationary machine systems use conveyors for moving bulk materials from place to place. Different types of conveyors are known, such as belt conveyors including endless canvas, rubber or metal belts which support the material being moved and are pulled over pulleys or rollers, chain or cable conveyors which include chains or cables adapted to pull plates, buckets or containers loaded or filled with material being moved, and auger or screw conveyors which include a helix formed about a turning shaft for moving material through a tube. 
     Mobile machine systems which use conveyors include various types of agricultural vehicles and construction equipment. Combines, for example, typically include a clean grain elevator for moving material up to a delivery auger, a delivery auger for moving the clean grain into the grain bin, a tailings auger for moving tailings to the tailings elevator to be returned to the threshing system of the combine, and an unloading auger for moving material from the grain bin to a transport device. Other examples include cotton pickers having a conveyor for moving cotton into storage bins, planters having a conveyor for moving seeds or fertilizer, and forage harvesters having a conveyor to move foraged material. Some conveyors include pneumatic delivery systems which are used, for example, to deliver seed from a seed bin to a planter or to convey forage from a forage harvester to a wagon. Stationary systems using conveyors include, for example, grain elevators using a conveyor including a driven chain which pulls paddles loaded with grain. 
     The machine systems described above may include real-time sensors and systems for measuring or monitoring parameters of material moved by the conveyors. These sensed parameters may include, for example, the yield or mass flow rate of material being moved by the conveyor, or the moisture content of the material. For example, yield and moisture sensors may be mounted to a grain auger of a combine to measure the mass flow rate and moisture content of grain flowing through the auger. 
     Known systems for measuring moisture may include capacitive sensors mounted in or on a fin which extends into the flow of material to measure the capacitance of the material. These systems extend into the flow of material so that the sensors can detect moisture despite their limited range. However, the intrusion into the flow of materials may cause certain materials, such as plant residue or sap, to build up on the sensors as contact is made with material being moved. The resulting build-up can cause the sensors to give inaccurate or erroneous readings. In addition, the intrusion of the sensors into the material may restrict or interrupt the flow of material, and the exposed fins and sensors are subject to mechanical wear and breakage. 
     Other measuring systems use capacitive sensors in a test cell which receives a small portion of the material flow diverted from the main flow. Such systems, however, require additional components and structures to divert the flow of material from the main flow and for the test cell, thereby increasing cost and decreasing reliability. Such systems may also suffer from build-up on the sensors since the material makes contact with the sensors. 
     Known sensors used to measure certain parameters of material being moved, such as yield or mass flow rate, may contain radioactive isotopes. These sensors may be subject to regulation concerning their sale and use since they are radioactive sources, thereby subjecting the user to the increased costs and paperwork associated with regulation compliance. The user is also exposed to the costs and risks generally associated with the use and management of radioactive sources. Other yield sensors generate signals when harvested grain hits a plate, the signals depending on both the amount of grain hitting the plate and the force at which the grain hits. These sensors may be inappropriate for measuring parameters of certain non-granular materials, such as forage, and may be difficult to integrate into a particular system. 
     Another problem with known systems for measuring parameters of a material includes the limited frequency response of such systems. Certain parameters of a material, such as type, mass flow rate, moisture content, density or other parameters, can be identified or measured by driving a sensing element with different frequencies and measuring the response of the material to each frequency. For example, one measuring system which uses a capacitive sensor in a test cell includes three fixed frequency generators which generate three fixed frequency signals and a multiplexer which sequentially applies the frequency signals to the sensor. The response at each frequency is then measured. This system, however, may be unable to provide required resolution over a given frequency range because of the fixed frequency signals. Moreover, expansion of this system to include a sufficient number of frequency generators to provide the required resolution over a given frequency range may be impractical because of the high number of frequency generators needed. Another measuring system includes a sweeping frequency oscillator which drives a capaciflector sensor. This latter system is able to generate more frequencies than the former system. However, when a dynamic system is being measured, the use of swept frequencies may introduce errors because the dynamic system may change over the time required to sweep the frequency signals. 
     SUMMARY OF INVENTION 
     Accordingly, the present invention provides an improved method and apparatus for measuring parameters of material. The parameters which can be measured include material type, moisture content, mass flow rate, density and other parameters. Parameters are measured by determining the frequency response of the material to multiple simultaneous frequencies. The frequency response can be determined over a wide frequency range with required resolutions without the need for a large number of frequency generators. The parameters are accurately measured even in dynamic systems wherein the values change over time. Material can be measured in test cells, or while being moved by conveyors such as augers, elevators or pneumatic conveyors. Different types of sensing elements can be used such as capacitive, capaciflector, resistive or inductive sensing elements. 
     One embodiment of the invention relates to a method for measuring at least one parameter of material including the steps of generating a plurality of frequency control signals corresponding to a plurality of frequencies, generating a plurality of frequency signals having frequencies selectable by the respective frequency control signals, combining the frequency signals to generate a combined frequency signal having a plurality of frequency components, applying the combined frequency signal as an excitation signal to a sensing element coupled to the material being measured, determining the frequency response of the material at each of the frequencies based upon output signals from the sensing element, and analyzing the frequency response of the material to determine the at least one parameter. 
     Another embodiment of the invention relates to an apparatus for measuring at least one parameter of material including a frequency generating circuit configured to generate a combined frequency signal having a plurality of frequency components selected in response to a plurality of frequency control signals, a sensing circuit coupled to the frequency generating circuit and including a sensing element coupled to the material being measured, wherein the combined frequency signal is applied as an excitation signal to the sensing element and the sensing element generates output signals based upon the frequency response of the material at each of the frequencies, a signal conditioning circuit coupled to the sensing circuit and configured to determine the frequency response of the material at each of the frequencies based upon the output signals from the sensing element, and a signal processing circuit to analyze the frequency response of the material to determine the at least one parameter of the material. 
     Another embodiment of the invention relates to a work vehicle including a support structure for supporting components of the work vehicle, a plurality of wheels coupled to the support structure to move the work vehicle on a surface, at least one of the wheels being powered to move the work vehicle along the surface, and at least one conveyor to move material from a first location to a second location on the work vehicle. The work vehicle further includes a frequency generating circuit configured to generate a combined frequency signal having a plurality of frequency components selected in response to a plurality of frequency control signals, a sensing circuit coupled to the frequency generating circuit and including a sensing element coupled to the at least one conveyor, wherein the combined frequency signal is applied as an excitation signal to the sensing element and the sensing element generates output signals based upon the frequency response of the material being moved at each of the frequencies, a signal conditioning circuit coupled to the sensing circuit and configured to determine the frequency response of the material at each of the frequencies based upon the output signals from the sensing element, and a signal processing circuit configured to analyze the frequency response of the material to determine the at least one parameter of the material. 
     Another embodiment of the invention includes an apparatus for measuring at least one parameter of material. The apparatus includes a noise generating circuit configured to generate a noise signal having a substantially even power spectrum across at least a range of frequencies, a sensing circuit coupled to the noise generating circuit and including a sensing element coupled to the material being measured, wherein the noise signal is applied as an excitation signal to the sensing element and the sensing element generates output signals based upon the frequency response of the material, a signal conditioning circuit coupled to the sensing circuit and configured to determine the frequency response of the material at multiple frequencies based upon the output signals from the sensing element, and a signal processing circuit configured to analyze the frequency response of the material to determine the at least one parameter of the material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts, in which: 
     FIG. 1 illustrates an agricultural harvesting vehicle including augers and elevators for conveying harvested plant material; 
     FIG. 2 is a partial cross-sectional view of an auger equipped with a non-intrusive sensor assembly located along a surface of the auger and configured to measure mass flow rate and moisture content of material moved by the auger; 
     FIG. 3 is a sectional view of the auger equipped with a non-intrusive sensor assembly taken along line  3 — 3  in FIG. 2 which includes a block diagram showing the coupling between the sensor assembly and associated electronic circuits; 
     FIG. 4 is a block diagram showing an elevator equipped with a non-intrusive sensor assembly such as shown in FIG. 3 located along a surface of the elevator and configured to measure mass flow rate and moisture content of material moved by the elevator; 
     FIG. 5 is an electrical schematic diagram which represents the sensor assembly and associated electronic circuits of FIG. 3 wherein the electronic circuits drive the sensor assembly, generate signals responsive to the dielectric value of the material, and process the signals into values representing parameters of the material; 
     FIGS. 6A-6C are electrical schematic diagrams which represent alternative embodiments of the sensor assembly shown in FIG. 5; and 
     FIG. 7 is an electrical schematic diagram which represents an alternative sensor assembly including a capacitive sensing element. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before proceeding to the Detailed Description, a general comment can be made about the applicability and scope of the invention. While the following discussion makes specific reference to a method and apparatus to measure parameters of plant material moved by conveyors on an agricultural harvesting vehicle, it should be understood that the present invention is more generally applicable to a method and apparatus for measuring parameters of any type of material in general. Thus, a system employing the elements recited in the appended claims, though used in other applications, is equally within the intended scope of the invention. 
     Referring to FIG. 1, a method and apparatus for measuring parameters of material moved by a conveyor are used, for example, by an agricultural harvesting vehicle  10  (e.g., a combine). Vehicle  10  includes a main body  12  supported by front drive wheels  14  to propel the vehicle, and rear steerable wheels  16  which may be unpowered or powered. Alternatively, a tracked propulsion system may propel vehicle  10 . A tool or implement  18  (e.g., a header) is movably coupled to the front of vehicle  10  to sever crop (e.g., wheat, corn) being harvested. However, other types of agricultural and construction equipment vehicles use other types of tools (e.g., row units, fertilizer spreaders, shovels, buckets) to transport bulk material in either direction between the vehicle and the ground or surface. The severed plant material is fed by a feeder  20  to an axial threshing and separating assembly  22  including a generally cylindrical casing  24  and rotor  26  mounted for rotation therein. A front-mounted impeller  28  on rotor  26  drives the material rearwardly, and the material is impelled helically about rotor  26  as rotor  26  rotates and cooperates with spiral vanes  30  located on an upper surface of casing  24 . 
     As the plant material moves rearwardly, a plurality of rasp bars  32  and transport bars  34  mounted on rotor  26  cooperate with a concave  36  and a grate  38 , respectively, to thresh the crop material such that most of the grain is separated and impelled outwardly through the concave and grate. Straw and other waste materials are impelled rearwardly from casing  24  by a rotary beater  40 . The separated grain falls onto a grain pan  42  and is conveyed rearwardly by an auger  44  for cleaning and collection. Grain pan  42  is a stationary member which supports the material moved by auger  44 . 
     A cleaning and collection system  46  separates grain from the waste materials (e.g., chaff, tailings and other wastes) received from auger  44  and grate  38 . System  46  includes a chaffer sieve  48 , a cleaning fan  50 , a grain sieve  52 , a tailings auger  54  and a clean grain auger  56 . Stationary members  55  and  57  support the material being moved by augers  54  and  56 , respectively. Chaffer sieve  48  separates the chaff from the grain and tailings by reciprocating in the fore-and-aft direction. Chaff unable to pass through openings in sieve  48  is carried away by an upward and rearward airflow from cleaning fan  50 . Grain and tailings passing through sieve  48  fall onto grain sieve  52  and are separated by reciprocations of the grain sieve. Tailings unable to pass through the openings in sieve  52  are moved rearwardly onto tailings auger  54  for disposal. Clean grain passing through both sieves  48  and  52  is collected by clean grain auger  56  and conveyed by a clean grain elevator  58  and a delivery auger  59  to a grain tank  60 . An unloading auger  62  within an unloading tube  64  is used to offload the harvested grain to a transport device (not shown). Power for the above-described crop processors is provided by the vehicle&#39;s engine (not shown). 
     Vehicle  10  is preferably similar to the 2100 Series of axial-flow combines made by Case Corporation except that any or all of the plant material conveyors are equipped with sensor assemblies  112  as disclosed herein. Vehicle  10 , however, could also include other conventional or axial-flow combines, cotton harvesters such as the model 2155 and 2555 cotton harvesters made by Case Corporation, sugarcane harvesters, hay balers, or other agricultural harvesting vehicles. A sensor assembly  112  is located on a surface of any or all of augers  44 ,  54 ,  56 ,  59 ,  62 , and an auger running across the length of header  18 , and elevator  58  or feeder  20 , to measure parameters of the plant material being conveyed. As explained below, the parameters can include the mass flow rate and moisture content of the material. The type of material may also be determined. Signals from sensor assemblies  112  can also be used to determine the rotational speeds of the auger screws in augers  44 ,  54 ,  56 ,  59  and  62 , and the speed of elevator  58  and feeder  20 . 
     Referring to FIGS. 2 and 3, an auger  100  includes a housing  102  and a screw  104  extending longitudinally and axially through housing  102 . Housing  102  can include a cylindrical tube such as tube  64  which encloses a screw  104  such as the screw of auger  59 . Alternatively, housing  102  may be a stationary member (e.g., flat or curved plate) running along the length of screw  104  but not enclosing screw  104  (e.g., grain pan  42 , member  55  or member  57 ). Screw  104  includes a shaft  106  and helical members  108  formed about shaft  106 . Shaft  106  is coupled to a power source (e.g., the vehicle&#39;s engine) through an appropriate gearing or transmission such that shaft  106  rotates within housing  102 . As shaft  106  rotates, helical members  108  advance loose or bulk material  110  such as grain, seed, forage, fertilizer, soil, etc. through housing  102  to move or convey the bulk material. 
     A sensor assembly  112  is located along an inner surface of housing  102 . Sensor assembly  112  is a non-intrusive sensor assembly which does not extend into the flow of material moved through auger  100 , and does not interfere with rotation of screw  104 . Sensor assembly  112  is preferably a capacitive-type sensor assembly with a structure related to a capaciflector proximity sensor assembly as described in U.S. Pat. No. 5,166,679, incorporated herein by reference. Thus, sensor assembly  112  is referred to as a capaciflector sensor assembly. The sensor assembly described in the &#39;679 patent detects proximity between a machine and an object with improved range and sensitivity as compared to other capacitance proximity sensors. These characteristics are provided by a shield conductor located between the sensor probe and a reference plane. Sensor electronics drive the shield with the same excitation voltage as the sensor probe to block the direct capacitance between the sensor probe and the reference plane. Since the electric field lines of the sensor probe are effectively focused away from the shield, a larger change in signal occurs when an object intrudes in front of the sensor as compared with traditional sensors. 
     Sensor assembly  12  preferably includes five layers of electrical conductors located along an inner surface  114  of housing  102 . Starting with the conductor closest to material  110 , sensor assembly  112  includes a sensor  116 , a sensor shield  118 , a compensation sensor shield  120 , a compensation sensor  122  and a reference plane  124 . As shown in FIG. 3, sensor shield  118  is preferably larger than sensor  116  to reduce parasitic capacitance between sensor  116  and plane  124 . Compensation sensor  122  provides a signal used for environmental compensation of the sensed parameter of material  110 , and compensation sensor shield  120  is larger than compensation sensor  122  to provide effective shielding. Reference conductor  124  provides a clean ground plane. Alternatively, if made of conductive material, housing  102  can form the reference plane provided the electrical noise level is low enough. Sensor assembly  112  may be located such that sensor assembly  112  is consistently covered with material  110 . 
     Conductors  116 - 124  are preferably made from strips of conductive material such as aluminum or copper, but can be made in other shapes and using other materials. In one embodiment, conductors  116 - 124  are made from strips of copper foil which can easily be made to conform to the contours of housing  102  regardless of the shape of housing  102  (e.g., a tube, a plane or another shape). An insulator (not shown) such as a polyimide material (e.g., “KAPTON”) provides insulation between adjacent conductors  116 - 124 , and between reference plane  124  and housing  102 . Sensor assembly  112  may also be fabricated using other methods of making conductive or non-conductive layers which are known to people of skill in the art. 
     A cover  126  is preferably placed over sensor  116  to separate sensor assembly  112  from material  110  flowing through auger  100 . Cover  126  is preferably a low-friction plastic plate, and may comprise a high molecular weight polyethylene. However, cover  126  may be made from other materials having a low dielectric value such that cover  126  is transparent to sensor assembly  112 . Cover  126  and housing  102  may be attached together using nut and bolt arrangements  128 , with optional spacers  130  used to separate cover  126  from housing  102 . Cover  126  may also comprise a coating (e.g., a urethane coating) cast over sensor assembly  112 , or may be formed by a variety of methods known to people of skill in the art. 
     Sensor assembly  112  operates by detecting the capacitance of sensor  116  relative to reference plane  124 . Sensor  116  forms a first electrode of a sensor capacitor and material  110  forms a second electrode of the sensor capacitor. The capacitance of this sensor capacitor depends on the dielectric value of material  110  moving through housing  102 . This value, in turn, depends on the dielectric of the material that is influenced by factors such as the mass of material in auger  100 , the moisture content of the material, and the type of material. Environmental compensation is provided by signals generated by compensation sensor  122 . 
     Conductors  116 - 124  of sensor assembly  112  are coupled to electronic circuits  132  via electrical conductors  134 - 142 , respectively. Electronic circuits  132  are described below in relation to FIG.  5 . 
     Other configurations of sensor assembly  112  may be used. For example, the dimensions of conductors  116 - 124  may be changed in either or both directions, and sensor assembly  112  may encircle the entire circumference of housing  102  to form a tube. Sensor assembly  112  may have a planar shape for an auger (e.g., auger  44 ) which moves material over a planar member (e.g., grain pan  42 ). 
     Referring to FIG. 4, an elevator conveyor  150  such as clean grain elevator  58  is equipped with non-intrusive sensor assembly  112  located along an inner surface  152  of elevator housing  154 . Cover  126  is placed over sensor  116  to separate sensor assembly  112  from moving members within elevator  150  which include a belt or chain  156  and fighting or paddles  158  attached to chain  156 . Power from, for example, the engine of vehicle  10  moves chain  156  in an endless loop in the direction of arrows  160 . Paddles  158  pick up a volume of bulk material  110  and convey the material upward past sensor assembly  112 . Sensor assembly  112  generates signals responsive to the mass flow rate, moisture content and type of material  110 . Further, the generated signals are responsive to the movement of paddles  158 . A similar arrangement is used to locate sensor assembly  112  along the bottom surface of feeder  20 . Such an arrangement is used to measure the mass of material flowing through the combine. 
     Referring to FIG. 5, electronic circuits  132  include a frequency generating circuit  200 , a sensing circuit  202 , a signal conditioning circuit  204 , and a signal processing circuit  206 . Each circuit is described below. 
     Frequency generating circuit  200  includes a plurality of frequency generators  208  which generate independent frequency signals  210 , amplifiers  212  which amplify signals  210  to generate amplified frequency signals  214 , a summing amplifier  216  which combines signals  214  to generate a combined frequency signal  218  having multiple frequency components, and a signal gain amplifier  220  which amplifies signal  218  to generate an amplified combined frequency signal  222 . Signal  222  is the output signal from frequency generating circuit  200  which is applied to sensing circuit  202 . 
     Preferably, each frequency generator  208  responds to its own control signal  224  to generate a frequency signal  210  having an adjustable frequency. Thus, each generator  208  can generate a frequency signal  210  having any of an unlimited number of frequencies. A number m (e.g., 4) of generators  208  generate m independent frequency signals  210  simultaneously, each of which is adjustable. Other numbers (e.g., 2, 3, etc.) of generators  208  may also be used. By increasing the number m of generators  208 , the frequency response of material  110  to more frequencies can be determined simultaneously by circuit  132 . Frequency generating circuit  200  could alternately include a white or pink noise generating circuit to generate a signal having many frequency components. 
     In one embodiment, generators  208  include digital frequency generator integrated circuits (ICs) to generate frequency signals in response to digital words written to the ICs as digital control signals  224 . For example, generators  208  may include 10-bit numerically-controlled oscillators such as AD9850 devices available from Analog Devices. Voltage-controlled oscillator (VCO) circuits may also be used. FIG. 5 shows signals  210  as sinusoidal waves. Alternatively, other signals (e.g., square waves) may be used if no other signals are present at the signal harmonics. Because only the signals&#39; primary frequency is of interest, the harmonics of non-sinusoidal signals can be identified and ignored in the power spectrum. 
     Amplifiers  212  condition frequency signals  210  to an appropriate level such that amplified signals  214  can be accurately combined by summing amplifier  216 , and so as to minimize the power dissipation of sensor assembly  112 . For example, even if summing amplifier  216  was capable of attaining the slew rate required for each individual signal, the slew rate required for the sum of the signals could exceed the maximum slew rate of summing amplifier  216 , thereby causing distortion. To prevent distortion, the level of each signal is adjusted by amplifiers  212 . Summing and signal gain amplifiers  216  and  220  sum and control the combined signal level of the signals. Amplifiers  216  and  220  can be combined into a single amplifier circuit. The gain and offset of amplifiers  212  and  220  can be adjusted by appropriate control signals  226  and  228 , respectively. Although referenced by common reference numbers in FIG. 5, control signals  226  and  228  are controlled separately for each amplifier in electronic circuit  132  to optimize signal level and power requirements for each frequency. 
     Sensing circuit  202  receives amplified combined frequency signal  222  from frequency generating circuit  200  and applies excitation signals to sensor assembly  112 . Signal  222  is applied to sensor  116  via conductor  134 , and is applied to a buffer circuit  230  including a unity-gain amplifier  232  for driving sensor shield  118  with low impedance via conductor  136 . Sensor  116  and sensor shield  118  are driven by a common source to negate capacitive effects between sensor  116  and sensor shield  118 , thereby focusing the electric field produced by sensor  116  towards material  110  and away from reference plane  124 , thereby increasing the sensitivity to material  110 . Further, signal  222  is applied to compensation sensor  122  via conductor  140 , and is applied to a buffer circuit  234  including a unity-gain amplifier  236  for driving compensation sensor shield  120  with low impedance via conductor  138 . Sensing circuit  202  includes reference plane  124  which is coupled to ground via conductor  142 . Output signals from sensing circuit  202  are applied to signal conditioning circuit  204  and include sensed signal  238  and compensation signal  240 . 
     Signal conditioning circuit  204  includes a combined signal adjustment amplifier  242  to adjust the level of sensed signal  238 . An amplified sensed signal  244  is generated by amplifier  242  and is applied to a plurality of band-pass filters (BPFs)  246 . BPFs  246  filter signal  244 , and selectively filter compensation signal  240 , and apply the filtered signals  248  to a plurality of filtered signal adjustment amplifiers  250 . The adjusted filtered signals  252  are digitized by a plurality of analog-to-digital converters (ADCs)  254  and the digitized values  256  are made available to signal processing circuit  206 . 
     Combined signal adjustment amplifier  242  adjusts the level of sensed signal  238  to maintain signal strength and low impedance for the signal entering BPFs  246 . BPFs  246  filter amplified sensed signal  244 , and compensation signal  240 , to ensure the Nyquist criterion is satisfied for the given conversion rates of ADCs  254 . The Nyquist criterion requires that a digital signal be sampled at twice the highest frequency content of the signal. Thus, the upper cutoff frequency f Hn  for the nth BPF  246  is set to a maximum of one half the conversion rate for that ADC  254 . For example, for an ADC conversion rate of 200 KSamples/sec, the upper cutoff frequency for that BPF  246  is 200 KHz/2=100 KHz. The lower cutoff frequency f Ln  can be arbitrarily defined. However, if the DC component of signal  244  is desired, the lower cutoff frequency is set to 0 Hz to make a low pass filter. If it is desired to exclude the DC component from the power spectrum, the lower cutoff frequency is set to a nominal value to eliminate the DC component of signal  244 . 
     In one embodiment, the cutoff frequencies BPFs  246  and corresponding conversion rates of ADCs  254  are: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 BPF 1: f &lt; 1 KHz 
                 ADC 1: 2 KS/sec 
               
               
                   
                 BPF 2: 1 KHz &lt; f &lt; 100 KHz 
                 ADC 2: 200 KS/sec 
               
               
                   
                 BPF n: f In  Hz &lt; f &lt; f Hn   
                 ADC n: 2*f Hn  S/sec 
               
               
                   
                   
               
            
           
         
       
     
     Filtered signal adjustment amplifiers  250  control the gain of the signals output from BPFs  246  to achieve maximum signal amplitude resolution given the finite resolution of ADCs  254 . The gain and offset of amplifiers  242  and  250  can be adjusted by appropriate control signals  226  and  228 , respectively. ADCs  254  convert the analog signals  252  into digital signals for use by signal processing circuit  206 . As described above, the sampling rate of ADCs  254  is selected to be no less than twice the upper cutoff frequency of BPFs  246 . 
     Signal processing circuit  206  includes memory storage devices  258  which store the digitized signals received from signal conditioning circuit  204 , and a processing circuit  260  which processes the digitized signals to determine various parameters of material  110 . Processing circuit  260  includes an interface  262  (e.g., RS-485, J-1939, or other analog or digital interface) for communication with an external system (not shown). 
     In one embodiment, memory storage devices  258  include dual-port RAMs (DPRs) and processing circuit  260  includes a digital signal processor (DSP). For example, the ADSP-21060-LBW-160X DSP available from Analog Devices may be used. However, other types of memory devices or processing circuits (e.g., other DSPs or microprocessors having sufficient throughput) may be used. The amount of memory in each device  258  depends on the conversion rate of ADCs  254  and the desired frequency resolution: 
     
       
         Memory (samples)=Conv Rate (Hz)/Freq. Res. (Hz) 
       
     
     For example, with a conversion rate of 2 KSample/sec and a desired frequency resolution of 0.5 Hz, memory storage device  258  for this frequency is required to store 4 KSamples. DRPs  1  and  2  in FIG. 5 each store 4 K samples. 
     Preferably, processing circuit  260  (i.e., the DSP) generates digital control signals  224  which are applied to generators  208 , and generates gain and offset control signals  226  and  228  which are applied to amplifiers  212 ,  220 ,  242  and  250 . Each control signal  226  and  228  is generated independently to optimize signal level and power requirements for each frequency. Each control signal  224  is also generated independently to select the frequencies of interest. Control signals  224  may depend on input signals from an external device (e.g., computer) coupled to interface  262 . Thus, the external device may select the frequencies used to analyze material  110 . 
     In another embodiment, any or all of generators  208  and amplifiers  212 ,  220 ,  242  and  250  are not adjustable, and the corresponding control signals are not needed. 
     Processing circuit  260  performs a frequency analysis on the digital signals stored in memory circuits  258  to determine the frequency characteristics of material  110 . For example, processing circuit  260  generates a power spectrum by taking a Fourier transform of the digital signals. The power is determined as the square root of the sum of the real part squared and the imaginary part squared. The frequency characteristics are then used to identify various parameters of material  110  (e.g., mass flow rate; moisture content). The type of material and speed of the conveyor can also be determined. Because generators  208  are adjustable, any number of frequencies can be applied to sensor assembly  112  to determine the response of material  110  to any number of frequencies. Thus, a desired resolution can be achieved over a wide frequency range without requiring an impractically high number of fixed frequency sources. 
     Processing circuit  260  is configured to determine the power spectrum of sensed signal  238  by combining the power spectrums from the data acquired by each ADC  254 . The power spectrum is then used to determine parameters of material  110  being measured. By using multiple BPFs  246  and ADCs  254 , higher resolutions can be achieved at lower frequencies while still providing a power spectrum for a high bandwidth. Moreover, since the frequencies of interest are known, processing circuit  260  can also be configured to process only part of the power spectrum to obtain data only for the frequencies of interest. The partial power spectrum will eliminate the effects of any noise at other frequencies that are not of interest. 
     In one embodiment, processing circuit  260  determines the mass flow rate and moisture content of material  110  by applying the signals stored in memory circuits  258  to pre-determined processing algorithms. One or both of these parameters may be determined because the relative sensitivity of sensor assembly  112  to the mass flow rate and moisture content differ at different frequencies. A test and calibration process may be used to determine the algorithms. For example, the apparatus may be calibrated by recording test data detected by sensor assembly  112  as material is moved through auger  100  and comparing such test data to independently sensed or known mass flow rate and moisture content data. The test and known data may be input to a computer which is programmed to execute a curve-fit algorithm or statistical analysis package to curve fit the data. A neural network with a learning algorithm may also be used. 
     In one embodiment, processing circuit  260  executes a multiple linear regression algorithm to determine mass flow rate and moisture content using the equations: 
     
       
         mass flow rate= a   0   +a   1   f   1   +a   2   f   2   = . . . +a   n   f   n   
       
     
     
       
         moisture content= b   0   +b   1   f   1   +b   2   f   2   + . . . +b   n   f   n   
       
     
     wherein a 1  and b 1  are coefficients and f 1  are various frequencies. Coefficient values are determined using a calibration process and curve-fit algorithm as described above. Of course, a measuring apparatus such as that described herein may also be configured to detect only one parameter. The mass flow rate may be in units of kg/sec, and processing circuit  260  may accumulate or integrate mass flow rate to determine the total mass. Data generated from compensation signal  240  can be used to correct the parameters for environmental variations. 
     Processing circuit  260  may also be configured to determine the type of material based upon the sensor output in response to excitation signals of different frequencies. Thus, for example, processing circuit  260  may process the detected signals to distinguish between different types or varieties of grain. A calibration and testing process may be used to determine the algorithms used by processing circuit  260  to determine the type. 
     Processing circuit  260  may be configured to correct the calculated parameters for the effects of the rotation of auger screw  104 . The sensitivity of sensor  116  to the rotation of screw  104  may be affected by the completeness of the ring around tube  102 . When sensor  116  makes a complete ring, sensor assembly  112  may show little or no sensitivity to rotations of screw  104 , although variances in auger  100  may cause some sensitivity. However, when sensor  112  makes an incomplete ring, as shown in FIG. 3, the sensitivity may increase. To correct for the quasi-sinusoidal effect on the capacitance value of sensor assembly  112  as screw  104  rotates, a hardware filter may be used, or processing circuit  260  may be programmed to filter the input data. For example, circuit  260  may be programmed to average data over time, or to time sample the data such that the auger position is consistent. Similarly, processing circuit  260  may be configured to eliminate the effect of paddles  158  of an elevator. 
     Processing circuit  260  may also be configured to determine the rate of rotation of auger screw  104 . As screw  104  rotates, a quasi-sinusoidal signal or wave will be imposed on sensed signal  238 . To determine auger speed, processing circuit  260  may process the signal using a Fourier transform to determine a power spectrum, or the sensed signals may be conditioned with a high-pass filter and the frequency of the filtered signal measured using a comparator, counter circuit, or other techniques. Similarly, processing circuit  260  may be configured to determine the operating speed of elevator conveyor  150 . The velocity of material being conveyed by a pneumatic conveyor can be determined by measuring the fan speed and applying empirically-determined relationships between fan speed and air velocity. Fan speed can be measured, for example, using a rotational sensor coupled to the shaft driving the fan. 
     In one embodiment, a temperature sensor  264  is coupled to (i.e., attached to or embedded in) sensor assembly  112  to measure the temperature of the probe and to generate a temperature signal  266  read by processing circuit  260  through a signal conditioning circuit  268  which can include an A/D circuit. Temperature signal  266  is used by processing circuit  260  to temperature compensate the sensed signals. Temperature sensor  264  may include a thermocouple. 
     Electronic circuits  132  may be located in any suitable location on the particular conveyor system, and may be separated into several electronic packages. For example, circuits  132  could be attached to auger  100 , or the electronics which drive sensor assembly  112  could be located at the auger and the processing electronics could be located elsewhere. Processing circuit  260  can be part of another electronics package, such as a data processing unit on a combine, which performs other functions. 
     Referring to FIG. 6, three alternative sensing circuits  300 ,  302  and  304  which include capaciflector sensing elements are shown. In FIG. 6A, sensing circuit  300  includes a sensor  306  and a sensor shield  308  which are driven by combined frequency signal  222  via amplifiers  310  and  312 , respectively. In FIG. 6B, sensing circuit  302  further includes a reference probe including reference sensor  314  and reference shield  316 , which are also driven by signal  222  via amplifiers  318  and  320 , respectively. In FIG. 6C, sensing circuit  304  includes two sensor probes  322  and  324  and a common sensor shield  326  for both sensor probes. Probes  322  and  324 , and sensor shield  326 , are driven by signal  222  via amplifiers  328 - 332 , respectively. Probes  322  and  324  are preferably positioned at locations having differing relationships with the material being measured such that the sensitivity of the probes differ with respect to the parameters being measured. For example, probe  322  could be located along a side or bottom of an auger tube, and probe  324  could be located around the circumference of the auger tube. 
     The electronic circuits of FIG. 5 may also be coupled to other types of sensing elements to measure the frequency response of such sensing elements over a range of frequencies. For example, in FIG. 7, a sensing circuit  400  includes a capacitive sensing element  402  also driven by signal  222  via amplifier  404 . Sensing element  402  may form part of, for example, a capacitive cell which receives material to be tested. The cell can include a central cylindrical electrode and an outer concentric electrode configured to measure the dielectric of material placed between the central and the outer electrodes. In addition, the electronic circuits can be used to measure the frequency response of resistive or inductive sensing elements. For example, a resistive sensing element can be used to measure soil parameters (e.g., soil type, soil moisture content) by measuring soil resistance at different frequencies. Further, the electronic circuits can be used to measure both conductive and capacitive frequency response. The complex frequency response can be used, for example, to identify and analyze materials using various testing devices. 
     While the embodiments illustrated in the FIGURES and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. The invention is not intended to be limited to any particular embodiment, but is intended to extend to various modifications that nevertheless fall within the scope of the appended claims.