Abstract:
Methods and apparatuses are described for detecting volumetric moisture content and conductivity in various media based on a time-domain reflectometry (TDR) system wherein successive fast transitions are injected into a transmission line immersed in a medium of interest, and a characteristic received waveform is digitized and analyzed by continuously sampling multiple received waveforms at short time intervals. One method transmits a timing signal along a shielded transmission line while a coincident signal is transmitted through the medium of interest. Another method propagates the waveform along a transmission line, that may be either shorted or open-ended, and observes a reflected, rather than transmitted, waveform with a receiver connected to the same end of the transmission line as the transmitter. The effects of dispersion caused by the conductive and dielectric properties of the medium on the waveform in an unshielded transmission line are extrapolated by detecting the bulk propagation time and the slope of the distorted rising edge of the characteristic received waveform. Absolute volumetric moisture percentage is inferred from propagation time, and absolute conductivity is inferred from the maximum slope value of the distorted rising edge of the characteristic received waveform.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a Continuation-In-Part of U.S. patent application Ser. No. 09/945,528 that was filed on Sep. 4, 2001, (now U.S. Pat. No. 6,657,443, issued on Dec. 2, 2003. 
     This application is also a Continuation of, and claims the benefit under 35 U.S.C. 120 of the following two co-pending U.S. Patent Applications, both of which were filed on Feb. 19, 2003, and both of which are hereby incorporated by reference in their entireties into the present disclosure: 
     Application Ser. No. 10/367,688 titled “Method and Apparatus for Determining Moisture Content and Conductivity”, and 
     Application Ser. No.10/367,310 titled “Digital Time Domain Reflectometry Moisture Sensor.” 
     U.S. PATENT DOCUMENTS 
     U.S. Pat. No. 6,215,317 Apr., 2001 Siddiqui, et al 324/643 
     U.S. Pat. No. 6,441,622 Aug., 2002 Wrzeninski et al. 324/643 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     SEQUENCE LISTING OR PROGRAM 
     Not Applicable 
     FIELD OF THE INVENTION 
     The present invention relates generally to electronic moisture sensors, and specifically to time domain reflectometry moisture sensors. This invention represents modifications and extensions to the method and apparatus for extrapolating soil moisture and conductivity disclosed in U.S. patent application Ser. No. 09/945,528. 
     BACKGROUND 
     A variety of sensors have been developed to detect moisture in various media. These include conductivity sensors, bulk dielectric constant sensors, time domain reflectometer or transmissometer (TDR or TDT) type sensors, and various oscillator devices, the majority of which exploit the high dielectric constant of water to extrapolate moisture content in the medium. In particular, TDR type sensors have been used over the past several years to measure the water content in various applications. Such applications include detecting volumetric soil moisture, determining liquid levels in tanks, and determining moisture content in paper mills and granaries. 
     A major setback in determining volumetric moisture content in a medium is the influence of conductive materials in the medium of interest. For example, soil conductivity is a function of the ion content of the soil and of its temperature. Salts from irrigation water and/or fertilizer can build up in the soil and cause significant errors in TDR-based moisture readings. 
     Because of the uncertainty in moisture readings caused by conductivity, many of the TDR sensors now available are “relative” sensors. This means that the sensor does not report absolute moisture content readings, but uses reference points obtained through testing. In essence, the moisture sensor does not report absolute moisture content readings, but reports a “wetter than” or “drier than” condition based on the relative difference of the conductivity-dependent moisture content reading and the reference reading. 
     Unfortunately, the readings from these “relative” sensors do not remain in synchronism with the true or “absolute” water content of the medium, but fluctuate with time. For example, the salinity (ionic content) of soil may fluctuate with season. In such a case, the original reference point becomes an inaccurate indicator of the moisture level of the medium. 
     The method and apparatus disclosed in U.S. patent application Ser. No. 09/945,528 (&#39;528) provide a way to report absolute volumetric water content of a medium. This is done by essentially analyzing the distortion effects on a transmitted waveform caused by the properties (namely conductivity and dielectric constant) of the medium. The &#39;528 disclosure provides a means to launch a fast rising positive edge onto a transmission line passing through a specific length of soil. The previously disclosed embodiment and associated method may be modified to suit other configurations and implementations so as to more readily adapt the technique to other media in addition to soil. In a first set of alternatives, since the described system includes both a transmitter and receiver, some variations may be made in how the transmitter and receiver are physically related to one another within the moisture sensing system. A second set of alternative configurations, independent of the first, derives from variations in the manner in which the transmission line is terminated. 
     The embodiment described in &#39;528 uses a transmission line that folds back to a receiver mounted on the same circuit board as the transmitter. As a result of housing the transmitting and receiving electronics on the same circuit board, and folding the transmission line, feed-through noise is inherent in the characteristic received waveform. One possible variation from the previously described embodiment is to incorporate what may be referred to as a Bi-static approach. 
     With the Bi-static approach the transmitting and receiving circuitry are housed on separate circuit boards, connected by a straight unshielded transmission line used for sending the successive waveforms, a shielded transmission line used for timing, and a wire bundle for communication and power purposes. This eliminates the feed-through noise in the characteristic received waveform, resulting in a simpler detection scheme for bulk propagation delay and distorted rising edge slope. 
     Another alternative embodiment uses a reflected wave rather than the transmitted one. When using the reflected wave approach the transmitter launches a step function at one end of a transmission line, the other end of which may be either shorted or open-ended. The fast rising step function propagates along the line and is reflected at the shorted or open end back to the point of origin. A receiver samples and digitizes the returning waveform into closely spaced digital samples representing the amplitude at precise time intervals of the returned waveform. Analysis of these samples yields an accurate measurement of the round-trip propagation time of the step function, even in the presence of waveform distortion caused by conductive elements in the medium surrounding the transmission line. From the propagation time the bulk dielectric constant of the medium can be determined and from that the volumetric moisture content of the medium. Further analysis of the distortion of the waveform yields the bulk electrical conductivity of the medium. 
     BRIEF SUMMARY OF THE INVENTION 
     The disclosed invention is a method and apparatus for inferring volumetric moisture content and bulk conductivity of a medium of interest using a TDR-based system after the manner of the disclosure in &#39;528. The present invention describes alternative embodiments which use a Bi-static approach in one instance, and reflected wave approaches in other instances, to measure the propagation time. 
     In all embodiments as in &#39;528, a very precise timing and successive approximation amplitude-measuring scheme captures the timing of the received waveform with picosecond resolution and its amplitude with millivolt resolution. From point-by-point measurements, the characteristic received waveform is examined. Propagation delay of the characteristic received waveform is set as the first time when the amplitude of the received waveform is greater than a threshold. This information is used to infer the bulk dielectric constant of the moisture-bearing medium. The maximum slope of the characteristic received waveform is also examined and used to infer conductivity of the medium under test. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of a Bi-static sensor system. 
     FIG. 2 shows typical waveforms encountered in a Bi-static sensor system. 
     FIG. 3 is a simplified block diagram of a TDR sensor system. 
     FIG. 4 shows typical waveforms encountered in a TDR sensor system with an open-ended transmission line. 
     FIG. 5 shows typical waveforms encountered in a TDR sensor system with a shorted transmission line. 
    
    
     DETAILED DESCRIPTION 
     The particular apparatus disclosed in patent application Ser. No. 09/945,528 (&#39;528) may be modified in various manners. One modification physically separates the transmitting and receiving units; this will be referred to as the Bi-Static approach. Another independent set of modifications allow the transmitting and receiving units to be connected to the same end of the open or shorted transmission line, rather than to opposite ends. These are TDR (Time Domain Reflectometric) methods and will be discussed as such. For each such modified system, the method of extracting propagation delay and maximum slope are slightly different due to the inherent difference in the characteristics of the received waveform. 
     Bi-Static Approach 
     The important elements of a moisture sensor using the Bi-static approach are diagrammed in FIG.  1 . This figure is a simplified block diagram of a precisely timed waveform generator coupled with a successive approximation amplitude measurement system capable of capturing the detail of very fast waveforms. An electrical circuit schematic of one implementation of such a measurement system has been shown in &#39;528. 
     The timing generator  12  provides two trigger signals on outputs  20  and  21  that are precisely separated in time by a programmable offset ranging from zero to tens of nanoseconds with a resolution of tens of picoseconds. In the preferred embodiment the offset amount is governed by the setting of a first digital to analog converter (DAC)  14  as described in &#39;528. The author recognizes that the disclosed method is only one of many techniques for generating programmable time offsets. Other methods include but are not limited to programmable delay lines, programmable frequency synthesizers and programmable pulse width generators. 
     The first trigger output  20  activates a step function generator  13  that serves as a transmitter. The output of step function generator  13  is a very fast rising edge that propagates down an unshielded transmission line  16  to a latching comparator  19  that acts as a receiver. A second DAC  15  establishes a voltage reference level  22  to drive the other input of the latching comparator  19 . The delayed second trigger output  21  from timing generator  12  is sent down a shielded transmission line  17 ; the shield serving to isolate the signal from its surroundings so that the speed of propagation is independent of the properties of the medium of interest. 
     If the incoming waveform from the unshielded transmission line  16  is higher in amplitude than voltage reference level  22  at the time that the second trigger output  21  arrives via shielded transmission line  17 , then the latching comparator  19  provides a logical ‘1’ output. If the amplitude of the incoming waveform is lower than the voltage reference level  22  presented by the setting of the second DAC  15 , the latching comparator  19  provides a logical ‘0’ output. The state captured by the latching comparator  19  is examined by the microprocessor  11 . These features make it possible to measure the amplitude of the incoming waveform at a precise time after the waveform was launched. By repeatedly measuring the waveform amplitude at successive time increments, the entire waveform can be reconstructed. This reconstructed waveform is referred to hereafter as the characteristic received waveform. It will be noted though that the moisture-sensing methods described here do not require reconstruction of the entire waveform. 
     Measuring the amplitude of the characteristic received waveform at a given point in time is accomplished through a successive approximation technique requiring a sequence of waveform launch and receive cycles. The number of cycles required is equal to the number of bits of resolution in the voltage reference level DAC  15 . First, the microprocessor  11  sets the timing offset DAC  14  to establish a desired time delay between the two trigger outputs  20  and  21 . This setting represents the time after the launch of the waveform by step function generator  13  onto unshielded transmission line  16  at which the received waveform will be sampled at latching comparator  19 . This setting will remain fixed while the amplitude at this point is found. 
     Next, the voltage reference level DAC  15  is set to half scale, that is, the most significant bit is set and all other bits are cleared. Then an output from the microprocessor  11  starts the timing generator  12 . The first trigger output  20  from the timing generator  12  causes the step function generator  13  to launch a step onto unshielded transmission line  16 . At the precisely programmed interval later, the second trigger output  21  is sent down the shielded transmission line  17  and latches the input to the receiver, latching comparator  19 . It is noted that the latching actually occurs at the programmed offset plus the time required for the signal to travel down the shielded transmission line  17 , which is a known quantity. 
     Next, the microprocessor  11  examines the output of latching comparator  19 . If it is a logical ‘1’, as occurs when the transmitted waveform is higher in amplitude than the voltage reference level DAC  15 , then the microprocessor  11  leaves the bit most recently set in its set state and proceeds to set the next most significant bit. If latching comparator  19  indicates a logical ‘0’ output, due to the transmitted waveform being lower in amplitude than the voltage reference level DAC  15 , then the microprocessor  11  clears the bit most recently set before setting the next lesser significant bit. Then another step function is launched onto unshielded transmission line  16 . This sequence is repeated until all bits in the voltage reference level DAC  15  have been successively processed from the most significant to the least significant. The resultant input setting to the voltage reference level DAC  15  is the digital representation of the waveform amplitude at the precise time that was loaded into the timing offset DAC  14 . 
     FIG. 2 represents waveform measurements taken at successive time increments using the aforementioned process. The transmitted waveform  31  represents the output from the step function generator  13 . Received waveform  32  represents the portion of the characteristic received waveform that has propagated through moist soil, or another medium, that has low conductivity. Note that received waveform  32  is essentially the same as transmitted waveform  31  except that it has been translated to the right, that is, delayed in time, and its amplitude is slightly lower. Note that in the apparatus described in &#39;528, a low level signal preceded the transition in the characteristic received waveform, indicating residual feed-through due to the transmitter and receiver residing on the same circuit board. In this present disclosure using the Bi-static approach, no feed-through is observed since the first signal component to reach the latching comparator  19  (of FIG. 1) at the receiving end is the waveform that was sent down the unshielded transmission line  16 . 
     Received waveform  33  represents the characteristic received waveform that has propagated through moist soil, or another medium, having high conductivity. Note that received waveform  33  differs from received waveform  32  in that the rising edge slope is not as steep. However, the propagation times are nearly identical. This is expected since received waveforms  32  and  33  represent characteristic received waveforms that have propagated through media of equal wetness, but different conductivities. 
     For a given characteristic received waveform, the bulk dielectric constant and the conductivity of the medium of interest may be determined in a few ways. First, since there is no feed-through in the characteristic received waveform  32 , propagation may be inferred as that time when the amplitude of waveform  32  is greater than some threshold. This threshold may be set to a value above the noise floor of the receiving system and below a value that would cause significant error in the time to propagate through conductive media. 
     Second, the propagation time may be calculated by projecting the maximum slope of the waveform onto the zero-Volt line (x-axis). This point of intersection represents the estimated propagation time. As described in &#39;528, the slope of the line having maximum slope can be used to infer conductivity. 
     A third way to determine propagation delay is by computing the second derivative. The major point of inflection corresponds to the bulk propagation time. 
     Since it is desirable to know the maximum slope in order to calculate conductivity, the authors have chosen to implement the second of the above methods. This method is also advantageous since the point at which maximum slope occurs is when most of the energy of the transmitted waveform is reaching the receiving end, hence at this point the greatest signal to noise ratio occurs, assuming stationary noise statistics. The slope amplitude (Volts/second) and temporal position (seconds) are accurate and repeatable. 
     The maximum slope of the characteristic received waveform is located in the following manner. Since we expect that the characteristic received waveform will contain noise, a first derivative approximation is incorporated to provide smoothing. To approximate the derivative at each point, a thirty-two point window of data is stored. The first derivative approximation at a point in the center of the window is calculated as the sum of the second sixteen entries minus the first sixteen entries, divided by the sum of all thirty-two entries. 
     A search for the maximum slope begins at a time when the characteristic received waveform is greater than some voltage above the waveform. The maximum slope, its temporal location, and the amplitude at that location are stored. Propagation time is then determined by projecting the maximum slope line  34  onto the zero-Volt baseline (x-axis) and noting the point of interception. Following the same procedure in a more conductive media will give the maximum slope line  35 , which intercepts the baseline at the same point, indicating a consistent propagation time without regard to conductivity. 
     TDR Methods 
     As shown in FIG. 3, the important elements of a moisture sensor using a TDR method are very much the same as shown in FIG. 1 for the Bi-Static approach, but reorganized slightly. The timing generator  12  again provides two trigger outputs  20  and  21  which are precisely separated by a programmable time offset, governed by timing offset DAC  14 , ranging from zero to tens of nanoseconds with a resolution of tens of picoseconds. 
     The first trigger output  20  activates a step function generator  13 . The output of this step function generator  13  propagates down transmission line  18  to the distal end where an optional shorting bar  23  may be installed. Whether transmission line  18  is open-ended or shorted, the signal will be reflected and returned to the receiver, latching comparator  19 . The primary difference between the TDR method and the above-described Bi-Static approach is that the latching comparator  19  is connected to the transmission line  18  at its proximal end, the same end that is driven by step function generator  13 , rather than at its distal end. The second trigger output  21  is applied to the latch input of the latching comparator  19 . 
     Depending upon whether the waveform amplitude at the driving and receiving end of the transmission line  18  is higher or lower in amplitude than the voltage reference level DAC  15  driving the other input at the time of the second trigger output  21 , the latching comparator  19  will provide a logical ‘1’ or logical ‘0’ output, respectively. The state captured by latching comparator  19  is then examined by the microprocessor  11 , which adjusts the voltage reference level DAC  15  and launches successive step functions until the amplitude of the characteristic received waveform at the time of the second trigger output  21  has been acquired. Then the timing offset DAC  14  can be adjusted to move the second trigger output  21  to the next time increment so that the amplitude at that point in time can be digitized. As in other described configurations, repeated measurements of the waveform amplitude at successive time increments allow the entire characteristic received waveform to be reconstructed, though it need not be. 
     Measurement of the amplitude of the characteristic received waveform is accomplished through a successive approximation technique similar to that described above for the Bi-static approach. In the TDR method, however, the fact that the receiver, latching comparator  19 , has been relocated from the distal to the proximal end of the transmission line  18  means that the second (shielded) transmission line ( 17  of FIG. 1) has been replaced by a direct connection between the second trigger output  21  from the timing generator  12  and the latching comparator  19 . 
     FIGS. 4 and 5 represent waveform measurements taken at successive time increments using the aforementioned process. In FIG. 4 transmitted waveform  41  represents the digitized waveform appearing at the driving/receiving end of an open-ended transmission line after a step function has been transmitted. The right-hand portion of the received waveform  42  represents the portion of the characteristic received waveform that has propagated through the moist media, reflected off of the open distal end of the transmission line  18  (FIG.  3 ), and returned to the point of origin. Note that this segment of received waveform  42  is a positive rising segment. For a shorted, rather than open, transmission line, refer to FIG. 5 where received waveform  52  with a negatively sloped segment is shown in response to transmitted waveform  51 . Either case applies in this disclosure. The amplitude and slope of the segment of interest in received waveforms  42  and  52  are affected by the electrical conductivity of the medium in which the transmission line  18  is immersed. The timing of the rise of the segment of interest in received waveforms  42  and  52  are determined by the bulk dielectric constant of the medium. Note that in the apparatus described in &#39;528, a low level signal preceded the waveform. This low signal represented residual feed-through due to the fact that the transmitter and receiver were housed on the same circuit board. When using the TDR method, the lead portion of the received waveform is identical to the transmitted waveform since the receiver is connected across the transmitter output terminals. 
     With increasing conductivity of the medium through which the characteristic received waveform propagates, the waveform will change from being like received waveform  42  to received waveform  43  in FIG. 4 for the case of an open-ended transmission line, or from  52  to  53  in FIG. 5 for a shorted transmission line. Note that received waveform  43  differs from received waveform  42  in that the rising edge slope is not as steep. However, the propagation times are nearly identical. This is expected since received waveforms  42  and  43  represent characteristic received waveforms that have propagated through soils or other media of equal wetness but different conductivities. Similar features will be noted in comparing received waveforms  52  and  53  with their negative sloping segments in FIG. 5 where received waveform  53  represents the signal returned through a medium that has higher conductivity. 
     The characteristic received waveform is analyzed to determine the bulk dielectric constant and the conductivity of the medium of interest through the following steps. First, the point of maximum slope of the reflected portion of the waveform is found from a mathematical analysis of the digitized waveform samples. This is done as in &#39;528 by taking the mathematical derivative of a moving average of successive samples and locating the point of the maximum derivative. The timing, slope and amplitude of that point are retained. Next, the approximate point of upward inflection of the segment of interest in received waveform  42  (or downward inflection of  52 ) is determined through a search for the maximum second derivative of successive digitized waveform samples. Once that point is found a search is made for a zero-slope waveform segment just to the left of (prior to) the inflection point. The amplitude at that point represents the baseline amplitude above which the reflected wave rises (FIG. 4, line  44 ), or below which it drops (FIG. 5, line  54 ) depending upon whether transmission line  18  (FIG. 3) is open or shorted. The maximum slope, calculated earlier, is projected in line  45  (or  55 ) from its amplitude and timing coordinates onto this baseline  44  (or  54 ). The intersection of the slope  45  (or  55 ) with the baseline  44  (or  54 ) represents the propagation time. The slope (as in lines  45  and  55 ) of the segment of interest in received waveforms  42  and  52  can also be used to infer the conductivity of the medium. 
     The maximum slope of the characteristic received waveform is located in the same manner as described for the Bi-static approach. As mentioned in that earlier section, this method of extracting the salient features of the received waveform targets the peak of the transmitted energy being returned to the receiver, thus providing the greatest signal to noise ratio for accurate and repeatable measurements of the slope and timing. 
     The description here of alternate embodiments of a moisture sensor is in no way intended to suggest that these are the only embodiments available. It will be apparent to those of ordinary skill in the related arts that various combinations of the methods and configurations described here can be implemented in keeping with the intent of the disclosed invention and may have particular utility in some applications without departing from the spirit and scope of the invention as represented in the attached claims. Furthermore, the methods described for capturing, extracting and analyzing data are not meant to limit in any manner the application of the described invention. 
     Although the preferred embodiment has been generally described for measuring the moisture content and conductivity of soil, other embodiments may be adapted for similarly measuring those physical properties in many bulk materials. Some other media of interest where the present invention has been recognized as useful are grains held in storage as for cracking or milling, paper in its various forms whether during processing of the pulp or storage of the finished product in sheet or roll form, and lumber products in various forms. Other uses include the measurement of liquid levels in tanks, and the detection of moisture as a contaminant in fuel storage and engines.