Abstract:
A sensor to detect the presence of water or moisture in bulk materials includes a standard circuit board and a three element transmission line. The sensor electronics include an oscillator responsive to a direct current voltage supply which provides a square wave voltage signal. The sensor electronics further include a phase detector which detects the difference in phase between the reference square wave voltage signal and a signal provided to the transmission line buried in a medium. The phase difference is proportional to the dielectric constant of the medium surrounding the transmission line.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a method and apparatus to detect the presence of moisture, and more particularly, to a method and apparatus to measure water content in bulk materials such as soils and food products, by measuring changes in the dielectric constant of the material.  
         BACKGROUND OF THE INVENTION  
         [0002]    Several methods and devices have traditionally been used to measure the water or moisture content of bulk materials, such as soils and food products. One well-known technique is to measure changes in the dielectric constant of the medium being measured. The dielectric constant of water is approximately 80, the dielectric constant of soil minerals and organic matter is around 4, and the dielectric constant of air is 1. Accordingly, changes in water content of a particular medium will result in large changes in the dielectric constant of the medium, which can readily be measured.  
           [0003]    There are numerous specific examples where knowing the moisture or water content can be critical. For example, without limitation, the moisture content of soil can give information which is useful for conserving applied irrigation water or reducing ground water contamination. Moisture content measurements on stored grain can be used to prevent spoilage.  
           [0004]    A particular problem with measuring and monitoring moisture content of materials, particularly soils, has been the expense and sophisticated nature of the equipment used to measure or monitor the soil. Traditional devices for measuring moisture content in soils have been relatively large and very expensive to manufacture. Generally, many hundreds of dollars have traditionally been required to manufacture a single moisture measuring apparatus. This has traditionally made it cost prohibitive for those in agriculture, for example, to use multiple moisture content measuring devices in a field.  
           [0005]    An explanation of use of a transmission line buried in soil to detect the presence of moisture in the soil by measuring the travel time of an electrical pulse in the transmission line is found in the publication entitled  Evaluation of simple Transmission Line Oscillators for Soil Moisture Measurement,  20 COMPUTERS AND ELECTRONICS IN AGRICULTURE (1998), pp. 31-44, authored by Gaylon S. Campbell and Russell Y. Anderson, which is incorporated in its entirety by this reference.  
           [0006]    Various other methods and apparatus exist for detecting the presence of moisture in porous materials. For example, many devices, such as that shown in U.S. Pat. No. 5,148,125, determine the presence of moisture in a material by measuring the propagation delay of an AC signal applied to a transmission line buried in the material. Systems such as these suffer from the disadvantage that, for operation, a user must have access to both ends of the transmission line.  
           [0007]    In view of the foregoing, there is a need to provide a moisture detection apparatus and method which will be relatively inexpensive to manufacture, easy to use in the field, and provide accurate data concerning the moisture content of porous materials.  
         SUMMARY AND OBJECTS OF THE INVENTION  
         [0008]    An object of the invention is to measure moisture content of bulk materials.  
           [0009]    Another object of the invention is to utilize time domain reflectometry (TDR) to measure the moisture content of bulk materials.  
           [0010]    Still another object of the invention is to utilize frequency domain methods (FD) to measure the moisture content of bulk materials.  
           [0011]    Yet another object of the invention is to provide a device for measuring the moisture content of bulk materials which is inexpensive to manufacture.  
           [0012]    Another object of the invention is to provide a device for measuring the moisture content of bulk materials which is easy to use in the field and can be readily installed at various locations.  
           [0013]    Another object of the invention is to provide an apparatus and method for detecting moisture content which utilizes a constant frequency and measures a phase lag to determine the dielectric constant of the medium in which the water content is being measured.  
           [0014]    Still another object of the invention is to utilize a single integrated circuit chip to produce oscillation, buffer a signal, and detect the phase while producing a voltage output.  
           [0015]    Another object of the invention is to use a circuit board both to route signals and secure circuit components, as well as to form the elements of the moisture sensor.  
           [0016]    The foregoing objects are achieved by the present invention which provides a sensor for detecting the presence of water or moisture in bulk materials. The sensor comprises, in one embodiment, a standard four-layer printed circuit board assembly consisting of two printed circuit boards bonded together. The circuit is etched in the top layer of the top board and a three element transmission line is etched in the second layer of the same board. A second P.C. board, identical in size and thickness to the first, but with no copper cladding, is bonded to the bottom side of the first board so the transmission line is insulated from the medium in which it is placed. An overmold seals and protects the circuit components, which are mounted on one end of the board on the top layer.  
           [0017]    The sensor electronics of the subject invention include an oscillator that is responsive to a direct current voltage supply for providing a square wave voltage signal. The sensor electronics also include the above-mentioned transmission line. The transmission line input is coupled to receive the square wave voltage signal. A phase detector is coupled to detect the difference in phase between a reference square wave voltage signal and the signal provided to the transmission line. The phase difference is proportional to the dielectric constant of the medium surrounding the transmission line.  
           [0018]    Other objects, features, and advantages of the invention will become apparent from the following detailed description of the invention with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    Preferred embodiments of the invention are described below with reference to the accompanying drawings:  
         [0020]    [0020]FIG. 1 is a perspective view of a moisture sensor according to the present invention;  
         [0021]    [0021]FIG. 2 is an exploded perspective view of the moisture sensor of FIG. 1;  
         [0022]    [0022]FIG. 3 is a block diagram of the moisture sensor electronics;  
         [0023]    [0023]FIG. 4 is a schematic diagram of the essential electronic components utilized in connection with the present invention; and  
         [0024]    [0024]FIG. 5 is a diagram showing various output voltages utilized and measured in connection with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The present invention comprises an improved moisture sensor  10 , illustrated in FIGS. 1 and 2. The moisture sensor  10  generally comprises a four layer printed circuit board  12 , that is shaped as a thin, flat probe to facilitate its use in probing materials. An overmolded plastic enclosure forms an electronics compartment or housing  14 . The electronics compartment  14  is constructed for housing electronic components as will be discussed more fully below. The moisture sensing apparatus  10  further includes a distal end  13  which forms a transmission line sensor. The transmission line is insulated from the medium by the non-conducting P.C. board material, reducing errors in high salinity environments.  
         [0026]    [0026]FIG. 2 shows an exploded perspective view of the sensor  10 . Moisture sensor  10  comprises first and second P.C. board layers  20  and  22 , each having a similar size and shape so that they can mate with and be bonded to one another. A three element transmission line  46 , is etched in the copper of the underside of the first layer  20 , and is insulated from the measurement medium on the upper side by board  20  and on the lower side by board  22 .  
         [0027]    Sensor electronics  50  are preferably mounted on the top of the circuit board, which forms part of the first layer  20 .  
         [0028]    It should be understood that in a preferred embodiment FR 4 type material may be used for each of the layers  20  and  22 . FR 4 comprises typical circuit board material comprising a composite fiberglass and epoxy material. An epoxy paint may be utilized to coat the outer layers of  20  and  22 . Other suitable materials may also be used.  
         [0029]    An illustrative block diagram of the sensor electronics  50  is provided in FIG. 3. Therein, the sensor electronics  50  includes an oscillator  52  for providing a square wave voltage signal. The oscillator  52  is coupled to the transmission line  46  for providing the square wave voltage signal thereto. A phase detector  54  is coupled to the oscillator  52  and the transmission line  46  to detect the difference in phase between the reference square wave voltage signal and the signal provided to the transmission line. The details of components included in each of these three blocks are shown in FIG. 4, along with a buffer block  53  which buffers the square wave signal from the oscillator to the transmission line.  
         [0030]    More particularly, as shown in FIG. 4, a suitable source voltage is provided by a voltage  56 , which is conditioned using first and second inductors  58 ,  60 , respectively, first and second capacitors  62 ,  64 , respectively, and a resistor  66 , to provide the source voltage at node  68  and at the power supply pin of the I.C. (not shown). As shown, oscillator  52  comprises first and second exclusive OR gates  70 ,  72 , respectively, which are configured as simple inverters, and with resistors  74 ,  76  and capacitor  78 , form an astable multivibrator.  
         [0031]    Configured as described, and with the proper selection of resistors  74 ,  76  and capacitor  78 , oscillator  52  will provide an output that is the square wave voltage signal having a desired frequency. Buffer  53  is a single exclusive OR gate  84 , configured as an inverter, which provides the square save signal from the oscillator  52  to the transmission line  46  and the phase detector  54 .  
         [0032]    Phase detector  54  has first and second inputs, which are operatively coupled to exclusive OR gate  80 . The output of the buffer  53  is a square wave voltage signal and is provided directly to the first input of the phase detector  54 . The second input of the phase detector  54  is coupled to the output of the buffer  53  through a third resistor  86 . The second input of phase detector  54  is also coupled to the transmission line  46 .  
         [0033]    The output of the exclusive OR gate  80  is coupled to resistor  90  and capacitor  92  which form a low-pass filter giving an output voltage  100  that is constant and proportional to the difference in phase between the inputs of the phase detector. The four exclusive OR gates shown in FIG. 4 are commonly available in a single I.C. such as the 74VHC86.  
         [0034]    [0034]FIG. 5 shows various example wave forms associated with the present invention. Reference numerals I through IV as shown in FIG. 5 correspond to those same reference numerals I through IV identified in FIG. 4. There are two wave forms shown at locations II and III, one for time domain reflectometry (TDR) and one for frequency domain (FD). The circuit will operate in either mode depending on the length of the transmission line and the speed of the exclusive OR gates  70 ,  72 ,  80 ,  84 .  
         [0035]    The voltage at I (FIG. 4) is a simple square wave, which is shown as wave I in FIG. 5. Wave I is the bottom input to the exclusive OR gate  80  (FIG. 4), while the top input to exclusive OR gate  80  is also operatively coupled to the moisture probe  46 . The wave form II will behave differently depending on the length of the probe. For a long probe (e.g., a ten-foot probe), the circuit will operate in a TDR mode where the wave at II will show a low plateau when the leading edge of the square wave reaches the start of probe  46 , and a peak plateau when it has reached the end of the probe and is reflected back to the start  46 . For a short probe (e.g., a six-inch probe), the probe  46  will be in a FD mode, and the voltage at II will act like a charging and discharging capacitor (II FD in FIG. 5).  
                                           TABLE 1                           Truth Table for Exclusive OR Gate 80            X   Y   Output                    0   0   0       0   1   1       1   0   1       1   1   0                  
 
         [0036]    Considering a long probe  46 , operating in a TDR mode when the wave I changes from low to high (at time A), wave I will be high while wave II (TDR) is still low as it is below its threshold voltage (i.e., the voltage required to change the state from “0” to “1”). According to Table 1, the “0” and “1” will result in an exclusive OR gate  80  output of 1, so the value of wave III at time A is “1.” At time B, wave II (TDR) increases to the peak value so both wave I and wave II (TDR) are now “1” and wave III is now “0.” The process reverses at time C when wave I becomes “0” and wave II (TDR) is still high. When wave II (TDR) drops below the threshold at time D, the output at wave III drops to “0” again. The exclusive OR gate  80  output at wave III charges capacitor  92  (FIG. 4) such that the voltage at wave IV remains nearly constant, only changing when the charge time (i.e., the time between times A and B and times C and D) increases or decreases. The charge time is directly related to the dielectric constant of the medium around the probe, which increases with high dielectrics and decreases with low dielectrics.  
         [0037]    The FD mode functions similarly to the TDR mode. The probe now acts as a capacitor, charging when the input from wave I is high, and discharging when the input is low (FIG. 5, wave II) FD. At time A, wave I is high, while wave II (FD) begins increasing in voltage from 0. The exclusive OR gate  80  output of wave III (FD) at time A is “1.” When the voltage reaches a threshold value at time B (indicated by horizontal lines on wave II (FD)), wave II changes from “0” to “1” and the output of exclusive OR gate  80  (wave III (FD)) becomes “0.” Again, when wave I returns to “0,” (FIG. 5, wave I), the process is reversed. The output of exclusive OR gate  80  has the same affect on the voltage at wave IV as the TDR circuitry.  
         [0038]    While this invention has been described with reference to certain specific embodiments and examples, it will be recognized by those skilled in the art that many variations are possible without departing from the scope and spirit of this invention, and that the invention, as described by the claims, is intended to cover all changes and modifications of the invention which do not depart from the spirit of the invention. The words “including” and “having,” as used in the specification, including the claims, shall have the same meaning as the word “comprising.”