Abstract:
An improved system and method for providing a dielectric monitor which allows the measurement of the dielectric constant of a conductive material. The capability to accurately and efficiently measure the dielectric constant in soil allows the moisture content of the soil to be accurately determined. The preferred embodiment teaches a sensor that has the ability to compensate for some level of variable conductivity. Alternate embodiments are applicable to areas other than soil moisture measurement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of, and claims priority from, application Ser. No. 11/163,423, filed on Nov. 3, 2005. 
     
    
     DESCRIPTION OF THE RELEVANT ART  
       [0002]     This application expands on and builds on the technology disclosed by DeHart in U.S. Pat. No. 6,798,215.  
       SUMMARY OF THE INVENTION  
       [0003]     It is the object of the present invention to provide a dielectric monitor which allows the measurement of the dielectric constant of a conductive material. The capability to measure the dielectric constant allows the moisture content of soils, where the sensor has the ability to compensate for some level of variable conductivity. The technique is also applicable to other areas beyond soil moisture measurement.  
         [0004]     The dielectric constant of a medium can be found by measuring the propagation delay of a wave traveling through that medium. The following formula gives the relationship between propagation velocity (V) and the bulk dielectric constant (k). C is the speed of light in a vacuum. 
 
 V=C/k   0.5  
 
         [0005]     Solving for bulk dielectric constant (k) 
 
 k =( C/V ) 2  
 
         [0006]     In an electrically conductive medium, the rise time of an electronic pulse traveling in a wave guide degrades because conductivity losses. DeHart in U.S. Pat. No. 6,798,215 taught a method of computing the amount of degradation and mathematically computing a correcting the propagation calculations based on determine the slope of the rising edge of the incoming wave. Anderson in Pub. No. US 2003/0042916 A1 teaches an alternative method of computing the amount of degradation and mathematically correcting the propagation velocity based on using a high speed latching comparator to sample the wave at a number of amplitudes. These samples effectively digitize the wave. The point of inflection marking the arrival of the incoming wave can be computed from the digitized wave. Anderson continues to refine and broaden this sampling technique in publications Pub. No. US 2004/0164750 A1, Pub. No. US 2004/0164746 A1 and Pub. No. US 2004/0059509 A1. While these methods are precise, they are typically complex and can be somewhat costly.  
         [0007]     This invention provides a method where the propagation time or velocity of the wave can be found in a way that is independent of the rise time of the incoming wave. This invention significantly extends the current state of the art by simplifying the detection the leading edge of the incoming wave. The incoming wave is differentiated to determine the point of inflection marking the arrival of the transmitted wave.  
         [0008]     This invention also provides a method to compute a measure of the amount of degradation of the wave, therefore inferring the amount of conductive material in the medium. Conductivity in soils is a marker for high ion content caused by salts and fertilizer. A high level of salts can cause changes in the preferred watering profiles. High levels of salt may also necessitate a remediation plan. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a diagram showing six time based wave forms. The first wave  100  represents a transmitted wave. The second wave  110  represents an ideal received wave with no degradation. The third wave  120  represents an actual received wave with degradation. The fourth wave  130  represents the wave after it is differentiated with its zero crossing points corresponding to the points of inflection  121  of the received wave. The fifth wave  140  represents the output of the comparator that toggles based on the polarity of the differentiated signal. This signal represents the true delay of the signal. The sixth wave  150  is the output of a circuit capturing the maximum value of the differentiated. The amplitude of this signal is inversely proportional to the signal loss.  
         [0010]      FIG. 2  is a block diagram of the preferred embodiment wherein the transmitted wave is transmitted down a transmission line  210 . The transmission line is placed in the medium of interest. The propagation time down the transmission line is a function of the characteristic impedance of the transmission line. The characteristic impedance is a function of the dielectric constant of the medium. The circuitry is designed such that as each wave reaches the end of the transmission line  235 , another wave is launched down the transmission line  215  causing a variable frequency that provides a useful estimate of the dielectric constant of the material. The output of the maximum amplitude capturing circuit  225  provides a method to determine the amount of conductivity material. The counter  275  counts the number of cycles in a period of time. An analog to digital converter in the processor  270  reads the maximum amplitude  226 . The processor  270  calculates the dielectric constant based on the number of cycles in a specific. The processor communicates the desired information through the system interface function  265 .  
         [0011]      FIG. 3  shows a comparison of a received wave  300  in a slightly conductive environment with the differentiated waveform  310  and the received wave  320  in a more conductive environment with the differentiated waveform  330 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]      FIG. 1  demonstrates the propagation delay of a wave traveling through a medium of similar dielectric constant but with differing conductivity.  FIG. 1  contains six time-based waveforms and is useful to illustrate the problem addressed by the present invention. The first wave  100  from the top represents an ideal transmitted wave. The next two waves,  110  and  120 , represent the received waves after the transmitted wave has been transmitted through the medium to be measured. The second wave  110  represents an ideal received wave. The third wave  120  illustrates a typical received wave in a medium with moderate conductivity. The slow rise and fall times of the second wave  120  illustrates signal degradation due to conductivity losses.  
         [0013]     The dielectric constant is estimated by determining the delay between the transmitted wave and the ideal received wave. Thus, if one can accurately determine the timing of the edges of the ideal received wave, one can accurately estimate the dielectric constant. However, due to limitations in inexpensive electronics used to process the signal, the received wave voltage cannot be measured directly to provide an accurate reading of the point of inflection of the received wave. Note that in the ideal case  110  the time difference between the arrival of the wave  111  and the point were it crosses the comparator trigger point  112  is very small. In the non-ideal case  120  the difference between the point of inflection reached at the arrival of the wave  121  and the point where the amplitude crosses the trigger point  122  is significant. This represents error and is significant in a conductive medium.  
         [0014]     The present invention solves this problem by accurately determining the point of inflection  121  in the received wave by differentiating the received wave. The output of the differentiator will approach zero  131  as the point of inflection is approached. After the point of inflection is reached, the received signal moves rapidly in with opposite polarity as is demonstrated between points  121  and  122 . The fourth wave  130  demonstrates the output of the differentiator when it differentiates the third waveform. The differentiator will move to a high voltage output  132  proportional to the maximum slope of the received wave. The differentiated signal has sharp edges that can be detected with a high-speed comparator. The fifth wave  140  represents the output of the comparator. Note that the comparator output matches the ideal wave  110 . The process of differentiation before the comparator effectively neutralized the effects of the conductive medium.  
         [0015]     The maximum amplitude  132  of the differentiated signal  130  is proportional to the rise time of the received wave. The maximum amplitude detector measures the peak amplitude of the differentiated voltage  151  and thus is an indication of the signal degradation. This parameter can be used as a second order correction factor when measuring the dielectric constant or output directly as a measure of the conductivity.  
         [0016]      FIG. 2  is a block diagram of a system implementing the correction method for processing the waves illustrated in  FIG. 1 . It is helpful to understand how the waves of  FIG. 1  are presented to the system of  FIG. 2 . Referring to both  FIG. 1  and  FIG. 2 , the transmitted wave, the first wave  100 , is the voltage at the sending end  215  of the transmission line  210  of  FIG. 2 . The ideal received wave, the second wave  110  shows the ideal wave as it arrives at the receiving end  235  where the impedance of the transmission line  210  matched producing an exact replica of the original wave  100 , but delayed in time. The received wave with moderate conductivity, the third wave  130 , shows the wave as it arrives at the receiving end  235  of the transmission line  210  of  FIG. 2  in a medium with moderate conductivity.  
         [0017]     In the moderately conductive medium, there is a definite rise and fall time associated with this wave  120 . Also note that the time it takes from launching of the transmitted wave to the zero crossing point  122  is definitely longer than the time to the zero crossing point  112  on the ideal received wave. The edge of the received wave corresponds to the point of inflection  121 . It is important to note that with moderate conductivity of inflection  121  arrives at the same time as the ideal wave  111 , but the fall time is significantly longer. The detection method senses this point of inflection, of the received wave,  111  and  121  at point  235 , by differentiating the incoming wave producing output  130  at point  220 . Note that the differentiator output  130  at point  220  crosses the zero crossing  135  at the point of inflection of the incoming wave  111  and  121 . The comparator  230  will output a positive voltage when the input  220  is a positive voltage and will put out a negative voltage when the input  220  is a positive voltage. The comparator  230  changes state as the input  130  at point  220  changes polarity producing the output  140  at point  215 . The output  140  of the comparator  230  changes state at the same time as the ideal wave  110 . The effects of the signal degradation caused by conductivity losses have been effectively cancelled out.  
         [0018]      FIG. 3  compares the performance of the technology in two samples with different conductivity.  FIG. 3  shows a comparison of a received wave  300  in a slightly conductive environment and its associated differentiated wave form  310  with the received wave  320  in a more conductive environment and its associated differentiated wave form  330 . Note that the amplitude  311  of the differentiated waveform  310  of the less conductive is greater than the amplitude  331  of the differentiated waveform  330 . This greater amplitude is a result of the faster rise time of the received wave  300 . This change of amplitude of the differentiated wave  311  and  331  is proportional to conductivity losses in the medium.  
         [0019]     The comparator will trigger at points  312  and  332 . Points  312  and  332  have very little time shift when compared to the time shift that would have been produced if the comparator triggered on the zero crossing points of the received waves  301  and  321 . The time shift between  301  and  321  is very significant and represents error in the propagation delay measurement. These curves  310  and  330  graphically demonstrate the results of using differentiation to find the arrival of the wave down the transmission line the allowing the true propagation delay to be easily determined even though the received waves  300  and  320  have very different characteristic shapes.  
         [0020]      FIG. 2  further illustrates a sensor apparatus composed of the following:  
         [0021]     A high speed analog differentiator  240   
         [0022]     A high speed comparator and line driver  230   
         [0023]     A maximum value capture circuit  225   
         [0024]     A transmission line  210   
         [0025]     A counter  275   
         [0026]     A control section composed of a microprocessor  270  with integrated analog to digital converter  
         [0027]     A system interface  265 .  
         [0028]     The high-speed analog differentiator  240  consists of U 1 , R 1 , R 2  and C 1 . A change of voltage at the input to the differentiator  235  causes a voltage change across capacitor C 1 , which intern causes a current to flow through C 1 . The relationship is defined by 
 
 I   cap   =C 1 *dV/dT  
 
         [0029]     Where I is the current through the capacitor, C 1  is the value of the capacitor, dV is the change in voltage across the capacitor and dT is the amount of time over which the change in voltage occurred. Basic operational amplifier design, U 2 , dictates that the current through resistor R 2  is equal to the current I cap . The Differentiator output voltage  220  is defined as: 
 
 V   out   =−I   cap   *R 2 =−C 1 *dV/dT  
 
         [0030]     V out    220  is the output of the differentiator  240 . V out  is the derivative of the voltage at the input  235  because the circuit performs the basic differentiation function of producing an output that is proportional the instantaneous change in the input voltage. In practice, a large value resistor is added in parallel with C 1 . This resistor provides the low frequency gain to initialize the system and assure startup. The value should be such that the current through the resistor is small compared the current through C 1  when the circuit is running at speed.  
         [0031]     The high-speed comparator  230  consists of U 2 , R 3 , R 4 , and R 5 . R 4  provides the reference for the switch point while R 3  and R 5  provide positive feedback providing noise rejection with crisp edges.  
         [0032]     An idealized maximum value capture circuit  225  is illustrated by Q 1 , R 6  and C 2 . As the differentiator output voltage  220  rises, the Q 2  will conduct causing the voltage  226  across C 2  to rise. When the differentiator output voltage drops, the base emitter junction will reverse bias and no current will flow. The voltage across the capacitor therefore retains the peak amplitude of the differentiator output voltage  220 . R 6  provides a discharge path that will eventually return the voltage across C 2  to zero. The value of R 6  is chosen such that the RC time constant of R 6  and C 2  is long compared to the time period of interest. This process is demonstrated in  FIG. 1  trace  150 . The trace at  151  demonstrates a portion of the cycle where Q 1  is conducting and the voltage across the capacitor is following the input voltage  220 . The trace at  152  demonstrates the portion of the cycle where the base emitter junction is reversed bias and the current flowing through R 6  is reducing the charge across C 1 .  
         [0033]     The output voltage of the differentiator  220  will go negative. When the differentiator voltage goes negative, the positive input to comparator  230  will go negative with respect to the negative input which in turn will cause the output  215  to go negative. The negative edge will now be transmitted down the transmission line  210 . When it reaches the end of the transmission line  235  the voltage at  235  stops rising and starts going negative defining a point of inflection where there is no slope and dV/dT=0. The output of the differentiator  220  will equal 0 volts immediately followed by a rapidly rising positive edge. The comparator switches just as the differentiator begins to move to a positive voltage. This process continues to indefinitely with the time between transitions equal to the propagation time of the delay line.  
         [0034]     A reading is initiated by a command arriving at the system interface  265  and presented to the processor  270 . The processor  270  exerts the clear line  276  on the counter. The processor  270  will then enable the counter  275  by asserting enable line  277 . Counter  275  will begin counting each time a positive edge is generated by the comparator  230 . The processor will assert the enable line  277  for a precise period of time and then de-assert the enable line  277 . The processor  270  will then read the total count through counter interface  278 . The processor  270  will also read the amplitude  226 . With this information the processor can determine the total distance traveled by the wave over the precisely timed period the counter  275  was enabled.  
         [0035]     The preceding embodiment discussed using a conventional delay line where a signal is imposed on the sending end of the transmission and the signal is picked up on the receiving end of the transmission line. Using a differentiator to determine the precise arrival of a received wave is equally applicable when used with time-domain reflectometry. A time-domain reflectometry system a wave is transmitted on the transmission line. The wave propagates to the open end of the transmission and is reflected back to the sending end of the transmission line. The reflected wave is coupled into the receiver. The transmission time is the time that it takes for the wave to propagate to the open end of the transmission and back.  
         [0036]     The disclosed technology is equally applicable to differential transmission line where both lines are energized with signals of opposite polarity.  
         [0037]     By knowing the total distance traveled by the wave in the give period the dielectric constant can be calculated. By knowing the number of times that the wave traversed the wave guide and subtracting out the electronic delay time and correcting for the conductivity of the medium, the total distance traveled by the wave in the sample time can be calculated. The distance traveled per unit of time is the propagation velocity. The following formula gives the relationship between propagation velocity (V) and the bulk dielectric constant (k). C is the speed of light. 
 
 V=C/k   0.5  
 
         [0038]     Solving for k 
 
 k =( C/V ) 2  
 
         [0039]     The moisture content can then be determined based on the dielectric constant.