Abstract:
A soil moisture probe includes a capacitance-type probe and a detection circuit. The probe includes two spaced electrodes on the same side of a printed circuit board (PCB). The electrodes are placed on an inner layer of a multi-layer PCB and the detection circuit may be placed on an outer layer. The PCB also includes a ground plane. The detection circuit generates a sawtooth or triangular wave which is converted to a DC voltage representative of the moisture content of a soil sample into which the probe is inserted. The unique circuit uses the capacitance of the probe as part of a low-pass filter that distorts an oscillator-generated square wave into a saw-tooth or triangular wave. A resistance component of the low-pass filter is adjustable, allowing tuning of the probe and the circuit as needed.

Description:
BACKGROUND 
       [0001]    Embodiments disclosed herein relate to soil science, hydrology and moisture detectors for use in soils, and in particular for a capacitance-based probe and a circuit for detecting moisture in soils, growing media, and other granular or fibrous media which may contain moisture. 
         [0002]    Plants require an adequate supply of water to grow well. Water content in media such as soil, sand and soil-less media can control many soil conditions, such as irrigation and fertilization conditions, surface runoff, erosion and salinity. 
         [0003]    Accurate measurement of water content in the root zone of plants is becoming ever more important as water resources become more limited in arid regions. As water becomes a more costly resource, growers are forced to cut usage back to the minimum required to grow healthy plants. Proper management of irrigation under these conditions requires measurement of soil moisture in the root zone. If too much water is applied, air is forced out of the soil and hinders root growth along with the obvious waste of water. If too little water is applied, plants become moisture stressed and fail to grow. The plants can even die if the water shortage is significant. If the moisture content of the soil is measured, both of these conditions can be avoided and healthy crops can be produced with a minimal amount of water. 
         [0004]    What is needed is a soil moisture sensor that is rugged, portable, and inexpensive, easy to clean, and able to handle a variety of soil samples in sequence. The moisture sensor should not be affected by contamination or traces from previous samples. It should be powered by a low voltage and consume minimal amounts of power when in use. 
       SUMMARY 
       [0005]    Embodiments described herein include a moisture sensor. The moisture sensor includes a capacitive moisture sensor and a detection circuit for measuring a moisture content of a sample with the capacitive sensor. The detection circuit includes an oscillator, a low-pass filter connected in series with the oscillator, the low pass filter including a resistance, wherein the capacitive sensor forms a part of the low-pass filter. The detection circuit also includes a peak detector connected in series with the low-pass filter. The peak detector includes a diode and a charging circuit, the charging circuit connected in series with an output of the diode, the charging circuit including a resistor and a capacitor, wherein the charging circuit is configured to charge the capacitor and wherein a charge on the capacitor is a function of the moisture content of the sample and an output voltage of the diode. 
         [0006]    Another embodiment is a moisture sensor. The moisture sensor includes a capacitive sensor with two electrodes formed on a same side of a printed circuit board and a detection circuit for measuring a moisture content of a soil sample with the capacitive sensor, the detection circuit formed on the printed circuit board. The detection circuit includes an oscillator and a low-pass filter connected in series with the oscillator, the low pass filter including an adjustable resistor, the capacitive sensor connected with an output of the adjustable resistor to form a part of the low-pass filter. The detection circuit also includes a diode connected in series with the low-pass filter and a charging circuit connected with an output of the diode, the charging circuit including a charging capacitor and a resistor, the charging circuit configured to charge the charging capacitor, wherein a charge on the charging capacitor is a function of the moisture content of the soil sample and an output voltage of the diode. The detection circuit also includes optionally a noise filter connected to the power supply. 
         [0007]    In one embodiment, the charging circuits used in the moisture detector may include one of (i) a power supply connected to a pull-up resistor and a charging capacitor connected to ground; and (ii) a charging capacitor and a pulldown resistor connected in parallel between the peak detector and ground. 
         [0008]    Another embodiment includes a method for detecting moisture in a soil sample. The method includes steps of sensing the sample with a capacitive sensor, generating a high-frequency square wave with an oscillator and sending the square wave through a low-pass filter to form a triangular wave. The method also includes a step of detecting a peak voltage of the triangular with a peak detection circuit, wherein the circuit passes only a DC voltage proportional to the peak voltage of the triangular wave and passing the DC voltage through a peak detector comprising a diode connected to one of (i) a pull-up resistor and charging capacitor; and (ii) a charging capacitor and a pull-down resistor, wherein a charge on the charging capacitor is a function of a moisture content of the soil sample. 
         [0009]    Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0010]      FIG. 1  is a perspective view of an embodiment of the soil moisture sensor. 
           [0011]      FIG. 2  depicts a first embodiment of a circuit diagram for operating the soil moisture sensor. 
           [0012]      FIGS. 3A-3D  depict signal levels at several point in the circuit diagrams of  FIG. 2  and  FIG. 4 . 
           [0013]      FIG. 4  depicts a second embodiment of a circuit diagram for operating the soil moisture sensor. 
           [0014]      FIGS. 5-6  depict an embodiment of the sensor for the soil moisture sensor. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The present patent discloses and describes our discovery, a soil moisture sensor that uses a capacitance probe. The rugged soil moisture probe described herein may be used with other circuits for generating an electric field and detecting moisture. The remarkable soil moisture circuit described herein may be used with other probes. Together, our new capacitance-based probe and the circuit form an inexpensive, reliable and effective tool for quickly determining a dielectric constant of a soil sample. From this measurement, a soil moisture is easily and readily determined. 
         [0016]      FIG. 1  depicts a soil moisture probe  10  with a capacitance-based sensor  12 , a handle  14  with a flexible portion  16  and a cable  18  for use with a moisture readout (not shown). As described herein, the sensor  12  is a multi-layer printed circuit board (PCB) with a suitable moisture-resistant coating. The handle  14  grasps the sensor  12  while protecting the circuitry within the proximal portion of the PCB. The rear portion of the handle may be flexible, allowing a range of movement for the cable. In one embodiment, the handle  14  is relatively stiff and is molded from a suitable thermoplastic material, such as polyamide or nylon, ABS or PVC. Elastomers may also be used, such as urethane or nitrile rubber. The flexible portion may be made from an elastomer or other preferred material. While details are not shown, those with skill in the art will appreciate that all interfaces in the probe  10  will be well sealed, both to prevent ingress of moisture into the handle and also to prevent buildup of soils or other samples on the outside of the sensor  12 . 
         [0017]    The sensor  12  forms part of the operating circuit depicted in  FIG. 2 . The operating circuit  20  of  FIG. 2  is easily placed on one side of a multi-layer PCB, such as a four-layer or four-surface PCB, used in one embodiment to form the sensor. Circuit  20  includes an oscillator circuit  22  with an oscillator U 1 . Oscillator U 1  in this embodiment runs at about 80 MHz. The circuit includes a 3V power supply and a capacitor C 1  as shown. The power sources depicted in  FIG. 2  may be supplied from a suitable battery or may alternatively be supplied from a remotely-located power source and routed to the points indicated in the circuit. 
         [0018]    The oscillator generates a square wave and sends the square wave at point A to an RC low-pass filter  24  formed by resistor R 1 , which may be a precision resistor, variable or adjustable resistor R 2 , and the capacitor formed by sensor  26 . An example of a square wave output by the oscillator at point A is depicted in  FIG. 3A . Low-pass filter  24  distorts the square wave into a sawtooth wave at point B, as depicted in  FIG. 3B  or  3 C.  FIG. 3B  depicts a waveform  200  for a sample that is relatively dry, i.e., very little moisture and very little distortion of the input square wave to the resulting sawtooth wave. In one embodiment, the square wave has a peak-to-peak voltage of about 3 V.  FIG. 3C  depicts a waveform  300  for a sample that is relatively wet, i.e. greater moisture and greater distortion of the input square wave to the resulting sawtooth or triangle wave. 
         [0019]    Selection of the values for R 1  and R 2  are based on the desire to maximize the difference in distortion of the square wave. The difference will be caused by the moisture in the sample and the resultant sensor readings, the frequency of the oscillator, and the capacitance of the sensor electrodes  62  and  64 , as shown in  FIG. 5 . Adjustable resistor R 2  allows calibration of the circuit  20  such that each sensor produced will have similar output voltages for given moisture conditions. Due to variations in component specifications and circuit board manufacturing tolerances, this calibration method is desirable to reduce unit to unit variation. Adjustable resistor R 2  is not strictly necessary for proper operation of the circuit, but it provides an easy way to increase the accuracy of the sensor. 
         [0020]    A larger value of sensor capacitance causes greater distortion of the wave and less peak-to-peak voltage. A lower value of sensor capacitance causes less distortion of the wave and greater peak-to-peak voltage. The capacitance of the probe is of course a function of moisture in the sample, i.e., moisture in the soil sample, with greater capacitance resulting from greater moisture content. As noted, an example of the waveform resulting from a relatively moist sample is depicted in  FIG. 3C . In this example, the sawtooth wave of  FIG. 3B  has been replaced with what might be called a triangular waveform. Those with skill in the art will recognize that the terms used in this disclosure to describe waves, such as square wave, sawtooth waves and triangular waves, are at best approximations. Such waveforms, as seen in  FIGS. 3A-3C , vary significantly from the geometric ideal of a square, a sawtooth, or a triangle. However, these are the terms used by people with skill in electronic arts and are intended in that sense. The distorted sawtooth wave may be detected at point B. 
         [0021]    The sawtooth wave is then sent to a peak detection circuit  28  for conversion. In the embodiment of  FIG. 2 , diode D 1  has a cathode connected to the sensor capacitor  26  and an anode connected to the output connector  38 . When the wave voltage present at point B is greater than the voltage at point D, no current flows through diode D 1 . When the wave voltage at point B drops below the voltage at point D, diode D 1  becomes forward biased and C 2  is discharged to through diode D 1  until the voltage at point D is equal to the voltage at point B plus the forward voltage of diode D 1 . As a result, the voltage at point D tracks the lower peak voltage of the wave form present at point B. When the voltage of the wave form at point B is greater then the voltage at point D, resistor R 3  adds charge to capacitor C 2  so that the charge is removed through D 1  when the lowest point of the waveform at point B is reached. 
         [0022]    The values of resistor R 3  and capacitor C 2  are selected such that the amount of charge that can be added to capacitor C 2  by resistor R 3  is relatively negligible during a single waveform cycle. Thus, there is effectively a DC voltage present at point D, the output of the peak detection circuit. In one embodiment as noted above, the sawtooth wave has a lower peak (or trough) voltage as shown at  300  in  FIG. 3C . In the embodiment of  FIG. 3D , the output voltage at point D is shown by line  301  for a wet sample and line  201  for a dry sample. Moisture in the sample adds capacitance and damps and distorts the sawtooth wave, converting it to a higher voltage. Thus, the output of the peak detection circuit  28  is proportional to soil moisture. 
         [0023]    This signal is then passed through a current-limiting resistor R 4  to an output on connector  38  for connection to a readout (not shown). Current limiting resistor R 4  stabilizes the signal when long cables are used between the connector  38  and the remote readout. The moisture sensor may also include a close-coupled noise filter  34  connected to the moisture sensor power supply. Noise filter  34  is primarily intended to remove 80 MHz noise from the oscillator that is present in the supply voltage and ground connections in the circuit. Noise filter  34  in one embodiment is a pair of 560 μH inductors  36 . 
         [0024]    The embodiment of  FIG. 2  is only one way of using the capacitance sensor described herein. Another exemplary circuit  40  is depicted in  FIG. 4 . The circuit of  FIG. 4  is very similar to that of  FIG. 2 , but in  FIG. 4 , the diode has the anode connected to the sensor capacitor, i.e., reversed from the previous example. In addition, the peak detection circuit uses a pulldown resistor R 3  in parallel with capacitor C 2 . Thus, circuit  40  has a peak detection circuit  48  that measures the higher peak voltage of the waveform present at point B. The higher peak voltage is depicted as peak  302  in  FIG. 3C . Therefore, a dry sample will produce a higher output voltage at point B than a wet sample. 
         [0025]    This embodiment also includes an oscillator circuit  42  with an oscillator U 1 , a low-pass RC filter  44 , the filter  44  including a fixed resistor R 1  (which may be a precision resistor) and a variable resistor R 2  in series, and connected to capacitance sensor  46  near point B. Peak detection circuit  48  includes capacitor C 2 , which is charged through diode D 1  during relatively high voltage signals of the waveform at point B. Resistor R 3  works to discharge capacitor C 2  when diode D 1  is reverse biased. The circuit of  FIG. 4  works in a way that is similar to the circuit of  FIG. 2 , but inverted. In  FIG. 4 , a DC voltage is present at point D, the voltage following the upper peak of the wave form at point B. The remaining portions of circuit  40  are similar, with current-limiter R 4 , and noise filter  54  with matching coupled inductors  56  and connector  58 . It will be understood that noise filter  54  may include other components, such as capacitors. However, close-coupled inductors are relatively small and effective while capacitors may have to be several hundred microfarads, and thus are not convenient for hand-held or portable use. 
         [0026]    A capacitance probe useful in the above circuit is depicted in  FIGS. 5-6 . However, other capacitance sensors may also be used with the circuits described above. In one embodiment, sensor  60  is a four-layer circuit board, that is two pieces of fiberglass and resin having four surfaces, two inner surfaces and two outer surfaces. The sensor includes two electrodes, an inner linear electrode  62  within a larger C-shaped electrode  64 . The outer electrode may also be described as a horseshoe electrode. Electrodes  62 ,  64  in this embodiment are approximately 1 ounce copper or copper alloy (about 0.0014 inches thick) on an FR-4 printed circuit board (PCB) as discussed above with respect to the circuits. Other thicknesses may be used. Other conductive metals or even other materials may be used. As depicted in  FIGS. 2 and 4 , inner electrode  62  is connected near point B in the circuits, while outer electrode  64  is connected to ground. The circuit board also includes control circuits  66 , discussed above with respect to  FIGS. 2 and 4 . 
         [0027]    The spacing between the electrodes is about 0.125 inches (about 3 mm). Inner electrode  62  is a little wider, about 0.170 inches (about 4 mm) and about 1.75 inches (about 4.4 cm) long, while outer electrode  64  is about 0.125 inches wide (about 3 mm) and about twice as long as inner electrode  62 . The circuit board is very convenient and portable, with an overall length of about 3 inches (about 8 cm), the wide or electrode portion about 2.65 inches (about 6.7 cm) long. The circuit board in this embodiment is narrow, about 0.75 inches (about 2 cm) wide. 
         [0028]    As shown in  FIG. 6 , sensor  60  includes a multi-layer circuit board with an upper layer  67  having two surfaces and lower layer  68  also having two surfaces. Upper layer  67  depicts inner electrode  62  and outer electrode  64  on a bottom side of the upper layer. Although not shown in  FIG. 6 , a control circuit, similar to one of  FIG. 2  or  FIG. 4 , is placed on the upper side  76  of upper layer  67  and connects to electrodes  62 ,  64  via internal connections between the top and bottom sides. Bottom layer  68  includes a ground plane  72  under only the portion of the circuit board containing the control circuit. After fabrication and assembly, the circuit board is coated with a solder resist or other thin, durable coating. This construction allows for convenient mass production of the sensor and the detection circuit. The resulting sensor/control circuit is sensitive to changes in the moisture content of its environment. At the same time, the circuit board is smooth and flat, with no seams, undercuts, or discontinuities in the distal or sensing portion. This makes the sensor easy to keep clean and to avoid interference between successive samples. 
         [0029]    There are many embodiments possible with this disclosure. For example, much more complicated ways may be devised to convert the sawtooth or triangular wave representing the output of the low-pass filter and capacitance sensor into a DC voltage. These may include multi-diode rectifiers or converters, op amps, and the like. However, at least one advantage of the present circuit lies in its simplicity, with the resulting reliability and low cost. Another advantage of this design also lies in its adaptability. Since there is inherent variation in manufactured components, adjustment of R 2  allows each sensor built to be adjusted to a similar output value and therefore reduces the variation from sensor to sensor. 
         [0030]    The electrodes discussed above may also be designed with different configurations. Since the probe is a capacitance probe, there will be two electrodes, forming the plates of a capacitor. There are many other ways of forming and placing the electrodes. The field depth of the electric field set up by the electrodes is somewhat proportional to the gap between the electrodes and may be varied. Thus, greater field depth may be achieved by extending the gap. Greater field depth may also be achieved by increasing the electrode areas for a greater penetration of the field into the soil or sample. There are tradeoffs, of course, since some of these other configurations may require greater power and larger physical size. 
         [0031]    It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.