Abstract:
An in situ ultra-low power contactless measurement apparatus and method suitable for micro-electronics in big data applications for continuously reporting a soil moisture profile at various zones.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to measurement devices and more specifically to measurement devices for measuring characteristics of a soil profile including volumetric water content, electrical conductivity and temperature at various depths in the soil. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention is primarily focused on taking accurate measurements of volumetric water content (VWC) in a variety of media, including but not limited to: soil, wood, concrete, building materials, clothing, and the like. This invention is useful in such applications as crop irrigation, lawn care, gardening and other landscaping applications, laundry systems, non-destructive testing for ground water infiltration into buildings, and a full range of moisture environments ranging from dry to completely saturated. 
         [0003]    The present invention is particularly suited for big data environments such as large scale deployment of numerous sensors such as industrial irrigation systems and municipal irrigation systems because the present invention addresses the issues of accuracy and total lifecycle cost of soil monitoring devices. The incident methods are intended for but not limited to synthesis into microchips. 
         [0004]    In relation to capturing a set of soil vital signs in a big data environment there is particular interest on the accurate measurement of volumetric water content in soil at large numbers of zones. However, this invention has wider applicability. 
         [0005]    The present invention sets forth a low-cost, ultra-low power moisture sensor and integrated communication system to make remote wireless applications possible which utilize continuous unattended monitoring of a multiplicity of locations or zones. Further, the present invention provides for the implementation of inexpensive sensors with improved longevity, durability, security, reliability and wide spread applicability. 
         [0006]    Measuring soil moisture content at a municipal or industrial scale is complex and includes a number of well-known problems in order to measure to an adequate level of accuracy and reproducibility at a reasonable cost. Often at the municipal and industrial level, deployment of a large number of sensors impacts the overall system cost because of the requirement to frequently replace batteries and the need to dean sensor probes to ensure adequate accuracy. In addition, the large number of sensors must work in a wide range of soil types and soil conditions from dry to fully saturated. 
         [0007]    There are a number of known techniques for measuring soil water content including (a) Neutron probe—uses radioactive material which is expensive and is typically inaccurate in topsoil (b) Matric potential—uses low cost gypsum which has slow response and lacks durability (c) Tensiometers—require regular maintenance (d) Time Domain Reflectometer—is expensive and is only accurate up to 65% VWC (e) Capacitive—is low cost but is susceptible to electrical conductivity issues (f) Frequency—low cost but is susceptible to electrical conductivity issues and is limited in range (g) Impedance Matching—expensive and limited range. 
         [0008]    The following references are representative of some of the known devices and techniques for measuring soil water content. 
         [0009]    U.S. Pat. No. 7,944,220, teaches the performance of a dielectric moisture content sensor which is commonly limited by sensitivity to salinity and nutrient levels in the soil, as well as sensitivity to temperature change. This is in part due to soil non-homogeneity and variation of soil composition. Most crops are grown in soil with a salinity and nutrient level corresponding to an electrical conductivity between 60 mS/m to 400 mS/m and as high as 500 mS/m to 600 mS/m for certain crops, such as tomatoes, with soil conductivity in coastal environments up to 3,000 mS/m. Temperature also creates large variations and must be considered in any viable method. Salinity and other nutrients are the primary cause of significant deviation and must be considered. The result of these issues it that most reasonably priced solutions do not function or measure VWC above 65% water saturation. 
         [0010]    U.S. Pat. No. 5,424,649, is representative of a common technique used in low cost sensors that have a thin dielectric coating. The dielectric coating is only partially effective at reducing sensitivity to soil conductivity which results in a moisture content sensor with sensitivity to soil conductivity and salinity. Coatings are also subject to wear and do not address the issue of charge stealing by the earth. 
         [0011]    U.S. Pat. No. 5,859,536 is representative of common techniques in more costly sensors which use impedance matching networks. Because these techniques depend on current flow into the earth, it is naturally susceptible to conductance of soil to ambient ground. 
         [0012]    U.S. Pat. No. 5,804,976 is a more reliable technique utilizing a transmission line and measuring propagation delay. However, this technique also suffers near total loss of signal at high saturation. 
         [0013]    U.S. Pat. No. 7,030,630 is a moisture sensor with a capacitive moisture measuring element and method of determining air humidity describes time constants associated with a parallel resistance but is silent regarding the parasitic series resistance. 
         [0014]    Current models and consequently the methods currently in use are effective for measuring humidity as in U.S. Pat. No. 7,030,630 but do not function in situ of greater that 65% volumetric water content material. 
         [0015]    U.S. Pat. No. 5,730,165 teaches that if the sensor employs an RC circuit or variation thereof, the stray conduction path will rob the plate of charging current and will thus alter its apparent time constant. 
         [0016]    Studies including the IEEE Experimental Electrical Modeling of Soil for In Situ Soil Moisture Measurement 2013 are instructive and provide a more accurate electrical model for soil over the entire range of saturation. This model, however, must be adapted to accurately represent the electrical parameters of sensors embedded in the earth. 
         [0017]    Often the previously known sensors require battery replacement each season which is a significant limitation to large scale deployment. 
         [0018]    Cost of the sensors and vulnerability of current sensors to tampering further prohibits wide deployment in unsecured areas. 
         [0019]    As a result, all of the current techniques have proven uses in particular segments but they suffer at least in terms of cost, complexity or accuracy, thus preventing ubiquitous adoption for large scale data collection such as in a metropolitan or industrial sensor network. 
       SUMMARY OF THE INVENTION 
       [0020]    The present invention is a volumetric water content sensor comprising an integrator circuit, switched capacitors and conductors placed in proximity of the material to be sampled. 
         [0021]    The signal applied to the sample is transient with very high frequency components. 
         [0022]    During each measurement only a portion of the transient response is captured by the integrator. 
         [0023]    Additionally, at least two separate measurements may be used in combination to determine volumetric water content and electrical conductivity. 
         [0024]    In this case, each measurement consists of generating a control signal with fixed period and different fixed duty cycle for a fixed number of cycles. 
         [0025]    During each measurement a different portion of the transient response is captured by the integrator making it possible to extract VWC and electrical conductivity (EC). 
         [0026]    One aspect of the invention is accounting for charge stealing by the earth in highly saturated soil. Considering in situ measurement using a three terminal model instead of two allows accurate measurement of 0% to 100% VWC media and true separation of the measurements of the permittivity and conductivity of the media, whereas conventional sensors cease to operate above 65% MC. 
         [0027]    A second aspect of the invention is measuring permittivity and EC by two measurements including a long and short sample window. A sample window is formed by timing the diverting of the electrical current between two separate sampling capacitors during successive charge events. A first measurement is made using a short window and second measurement is made using a long window. The window time is the percentage of time the first capacitor is charged compared to the charge event. 
         [0028]    This allows for a non-galvanic connection to the media which prevents oxidation issues due to exposed metal. It also prevents electroplating issues, thus extending the life of the sensor. Unlike traditional sensors it provides a full range measurement including dry to completely saturated media. 
         [0029]    A third aspect of the invention is aggregating thousands of individual measurement events per second in the analog hardware. This allows much greater sensitivity in completely dry son. It also provides accuracy as a tradeoff with time. The incident method typically provides 12 bits of accuracy in less than 2 milliseconds. 
         [0030]    A fourth aspect of the invention is the use of a transient measurement technique instead of an AC signal. This allows very high frequencies to be used resulting in measurements taken 100 times faster than traditional methods. 
         [0031]    The components, in contrast to traditional methods, do not include inductors or expensive oscillators and are suitable for synthesis into inexpensive microelectronics. 
         [0032]    A fifth aspect of the invention is that of detecting tamper by determining if the sensor has been unexpectedly moved or disturbed by, for example, a person or animal. 
         [0033]    The sensor may be housed in a durable waterproof enclosure and embedded in the earth or remotely mounted to a permanent structure such as a fence post or plant trellis. 
         [0034]    An example deployment is to directly burying the sensor in irrigation pipe trenches with antenna cable extending to sprinkler risers. 
         [0035]    The plurality of probes can be configured to include any number of zones with individual volumes of influence, including profiling the soil for adequate drainage and migration of nutrients. 
         [0036]    Comparisons with reference material inside the sensor removes the need for soil standards or recalibration. 
         [0037]    The low output impedance and low input impedance provided by this method provides an important mechanism for placement of the sensor electronics up to several meters from the probes by providing shielding up to the volume of influence for each of several zones. The external antenna connection provides further reach to above ground structures and puts the sensor out of sight. 
         [0038]    Because RF components are not used there is an associated reduction in the need for power. The reduced power consumption of the present application can provide four (4) updates per day for over six (6) years on AA batteries making it suitable for long term deployment; thus making it maintenance free for the entire lifecycle of the sensor. When combined with low cost metropolitan and industrial network radio microchips such as LoRaWan™ and SigFox™, it is suitable for public big data applications including thousands of zones over greater than a 15 Km radius and up to 1 Km in urban environments. 
         [0039]    The sensor may be configured to measure the moisture content of soil. However, the moisture content sensor may be arranged to measure other mediums such as wood, concrete, textiles, moisture sensitive polymers, etc. 
         [0040]    The probes may be constructed of nearly any conductive or semi conductive material and may be coated or left in contact with the media and include openings for moisture to pass freely. 
         [0041]    An embodiment of the apparatus and method may include the moisture content/EC sensor as well as a temperature sensor to measure the temperature of the media and surrounding media, humidity sensor, rain fall sensor, light sensor, wind sensor and the like for the prediction of future change in moisture. 
         [0042]    The invention stores reference profile data for the purpose of detecting removal or tampering of the sensor. It also includes cryptographic keys for preventing eavesdropping or man-in-the-middle attack, making it suitable for industrial and municipal applications. 
         [0043]    The invention is advantageous for tracking the factors for, and prediction of future moisture content in the media, thus enabling more judicious use of water and when coupled with precipitation forecast information it provides the platform for ubiquitous large data collection and efficient irrigation and prevention of water damage. 
         [0044]    In one embodiment, vertical arrangement of up to seven zones track the propagation of moisture in the media. This is advantageous to account for hysteresis and allow for creating critically damped volumetric water content in the media and prevent runoff or starvation of water. It also provides a mechanism for tracking the propagation of nutrient fertilizer through the soil. This information is used to optimize watering patterns and fertilizer patterns for various soil types and prevent undesirable runoff of bio-nutrients, which is a known cause of algae outbreaks in many lakes. 
         [0045]    Measurement of permittivity and electrical conductivity of the media is made by successive long and short window measurements either by capturing rising edges with a short window and falling edges with a long window or by performing a series of short window events followed by a series of long window events, or any combination thereof. This allows the sensor to function in completely dry media or when completely submersed in water. 
         [0046]    The measurement system is autonomous from the CPU. It is configured to execute N number of events then perform the analog to digital conversion and then activate the CPU. The CPU successively schedules long and short window measurements. It also schedules the other sensors such as temperature, light, humidity. After all of the data is collected, the CPU then activates the radio local oscillator and sends the data to the Internet repository, then notifies the power management timer to power off everything for for a period of time, six hours. 
         [0047]    A rapid measurement using only a very small number of events is performed periodically to detect tamper and perform rapid notification by storing and comparing against stored previous measurements. 
         [0048]    The features discussed in relation to any of the aspects of the invention may be applied to any other of the aspects of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0049]      FIG. 1  is a first embodiment with vertical profile arrangement of zones. 
           [0050]      FIG. 2  is a second embodiment with an alternative zone of influence. 
           [0051]      FIG. 3  is a surface profile arrangement of zones. 
           [0052]      FIG. 4  is a distributed zone arrangement. 
           [0053]      FIG. 5  is a schematic of a moisture sensor system. 
           [0054]      FIG. 6  is a graph containing example measurement data. 
           [0055]      FIG. 7  is an accuracy comparison of example measurements. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
       [0056]      FIG. 1  shows a volumetric water content sensor comprising an integrator circuit for measuring soil moisture content  1 . The VWC sensor measures moisture content at seven zones within the soil with each zone having a distinct volume of influence  19 . A volume of influence is the space within the soil in electrical proximity of probe  17  and probe  18 . The apparatus shown in  FIG. 1  also includes a radio and radio antenna  10 . The radio provides direct connection and logging of sensor data to a big data server via network including the Internet, WiFi, WiFi Max, LoRaWan™. The apparatus further includes a plurality of sensors for measuring other parameters which may be used in combination with the water content measurement for predicting soil conditions. The plurality of sensors may include: a topsoil temperature sensor  15 , a root temperature sensor  16  and an air temperature sensor  13 , a humidity sensor  12  and a light sensor  11 . The electronics for the moisture sensor are housed in an enclosure  14 . The enclosure is configured to be water proof. 
         [0057]    The electronics in this embodiment are housed with or adjacent to the moisture probes referring to Zone 1 Probe A denoted by  17  and Zone 1 Probe B denoted by  18 . Each set of probes placed adjacent or near each other form a continuous profile of moisture volumes of influence in the earth  20  over a certain depth. It should be noted that the probes may also be adjacent to the material being sampled and that the material may be any type of permeable media. 
         [0058]    Each zone is comprised of two probes. Multiple zones are arranged together to measure a VWC and EC profile at their respective depths or locations in the medium. 
       Second Embodiment 
       [0059]    The second embodiment shown in  FIG. 2  includes the means for measuring soil moisture content also at several levels within the soil and includes the plurality of sensors. The probes in this embodiment, however, are spaced horizontally and provide for a larger volume of influence by further separating the probes. Volumes of greater than a cubic meter can be sampled using the method of this invention. It is noted that previously available sensors measure up to only a few centimeters. 
       Third Embodiment 
       [0060]      FIG. 3  illustrates a third embodiment which includes a means for measuring topsoil moisture content at numerous locations. The probe conductors in this embodiment are spaced around a wheel such as on the wheel of a tractor or lawn mower. Samples may be correlated with GPS coordinates to create a detailed topsoil moisture map. 
         [0061]    Unlike traditional methods of moving a sensor above the surface or dragging a sensor across the material to be sampled, the current invention provides a consistent amount of pressure and contact with the soil. 
         [0062]    The current apparatus is useful to reduce the issues associate with air gaps between the soil and sensor probes. The current invention also applies to radio frequency backscatter measurement of volumetric water content as well as time domain reflectometry measurement of volumetric water content. 
       Fourth Embodiment 
       [0063]      FIG. 4  illustrates a third embodiment which includes a coaxial cable  35  connection between the various probes  36  and the enclosure  14  encasing the electronics. The driver shield  32  of the coax greatly reduces the parasitic capacitance of the coaxial cable  35 . Likewise the antenna  10  may be separated from the enclosure  14  using the antenna coax cable  31 . This allows placement of the probes under ground level in a high traffic area and hiding the electronics from view under the ground and allowing the antenna to be housed above or at the surface of the ground in a separate casing. 
         [0064]    The embodiments are to illustrate a few of the many possible configurations for measuring soil moisture content. It is also possible, for example, to define the zones to cover a physical volume, for example in a “square foot gardening” grow box or in separate plant containers or separate crop rows or to provide redundancy for accuracy. 
       Method 
       [0065]      FIG. 5  illustrates an electrical schematic of the moisture sensor system which provides the means of measuring soil moisture content. The electronics includes the following components:  41  CPU or radio System On Integrated Chip (SoIC);  42  Embedded Sensor Driver module;  44  Integrator;  45  first bank or switches or Quad DPQT CMOS Switch,  46  second bank of switches or Dual DPQT CMOS Switch;  47  Reference Impedance;  410  ( 411 - 416 ) Bank of Capacitors,  421 - 423  Voltage References. 
         [0066]    Control signals for the soil volumetric water content saturation are provided by Embedded Sensor Driver Module  42  of the CPU  41 . The Embedded Sensor Driver Module  42  outputs two (2) Pulse Width Modulated (PWM) signals used to control the first bank of switches  45  associated with capacitor bank  410  ( 411 ,  412 ,  413 ,  414 ), and the second back of switches  46  associated with capacitor bank  410  ( 415 ,  416 ). The Embedded Sensor Driver Module  42  also includes an Analog to Digital Converter (ADC) to capture the output of the Integrator  44 . 
         [0067]    The measurement sequence begins by the CPU  41 , creating two output signals, for example, a period of 4.4 microseconds. A first measurement, M01, is made using control signal  445 , for example, at 25 percent duty and control signal  446 , for example, with 50 percent duty cycle. 
         [0068]    The phase difference between signal  445  and signal  446  is made to be overlapping with signal  445  leading signal  446 , for example, a 0.3 microseconds overlap. This creates a sequence of alternating phase I and phase II cycles. 
         [0069]    During Phase I: Capacitor  411 , Capacitor  415 , Capacitor  412  and the probe are placed respectively in series with each other. Capacitor  416  is placed across ground Reference  423 , Capacitor  413  is placed across the +5 Volt Reference  421  and −5 Volt Reference  422 , and Capacitor  414  is conversely placed across −5 Volt Reference  422  and +5 Volt Reference  421 . 
         [0070]    During Phase II: Capacitor  413 , Capacitor  416 , Capacitor  414  and the probe are respectively placed in series with each other. Capacitor  415  is placed across the integrator and Ground Reference  423 , Capacitor  411  is placed across +5 Volt Reference  421  and −5 Volt Reference  422 , and Capacitor  412  is placed across −5 Volt Reference  422  and +5 Volt Reference  421 . 
         [0071]    The alternating sequence produces an alternating 20 Volt signal to appear across the probe terminals  17  and  18 . 
         [0072]    In this case, during Phase II the integrator sums the current collected in the probe during Phase I. This allows for a very short window for collecting the current in the probe during Phase I and a long time for the integration during Phase II. This allows for an inexpensive OpAmp, for example, &lt;1 MHz Gain Bandwidth. The resulting filter response however is capable of capturing frequency components &gt;5 MHz for vastly improved bound moisture detection. 
         [0073]    The CPU  41  continues this sequence for a specified number of cycles, for example, 0.25 milliseconds to allow the probe to reach a nominal average common mode voltage. 
         [0074]    The CPU  41  then releases the Reset  447  on the Integrator  44 . 
         [0075]    The CPU  41  continues this sequence for another specified or predetermined number of cycles, for example, a total of 256 cycles or 1.75 milliseconds longer. This produces a full range of 0 Volts for dry material to 2.5 Volts for 100% saturated material with very high precision in under 2 ms. For incredibly precise measurements or for extremely large volume of influence the number of cycles can be increased to approximately 5000 cycles limited only by the input referred offset of the OpAmp. 
         [0076]    The CPU  41  then stops both control signal  45  and control signal  446 . 
         [0077]    The CPU  41  then starts an Analog to Digital Conversion, ADC. 
         [0078]    The CPU  41  records the ADC measurement as M1. 
         [0079]    The CPU  41  then repeats the measurement sequence with control signal  45  and control signal  46  both at 50 percent duty cycle. 
         [0080]    The CPU  41  then records the ADC measurement as M02. 
         [0081]    The CPU  41  then signals the switch  46  to connect the probe terminals  17  and  18  to the Reference Impedance  47 . 
         [0082]    The CPU  41  then repeats the measurement sequence with signal  445  at 25 percent duty cycle and records the ADC measurement as M11. 
         [0083]    The CPU  41  then repeats the measurement sequence with signal  445  at 50 percent duty cycle and records the ADC measurement as M12. 
         [0084]    The CPU  41  then signals the switch  446  to connect the probe terminals to Zone(N) and repeat the same measurement sequence to record MN1 though MN2 for each zone. 
         [0085]    The CPU  41  then computes the raw saturation using the following steps. 
         [0086]    The measurements Mx1 and Mx2 are adjusted to remove the parasitic capacitance by subtracting M01 and M02respectively. 
         [0087]    The measurements are then scaled to account for temperature and supply voltage variation by multiplying by the scaling factor of M11 and M12 from the 50 percent reference  47 . 
         [0088]    Saturation for each zone is then extracted based on the ratio of the duty cycles. For example with 25% and 50% using the formula, Saturation=(2*Mz1−Mz2)*100%. 
         [0089]    Electrical Conductivity for each zone is then extracted using the formula EC=(2*Mz2−Mz1) 
         [0090]    Improved accuracy can be achieved by adjustments to the formula ratio as necessary based on actual measured window sizes for individual apparatus embodiments. Generally, simply using the control signal duty cycle ratio produces better than 2% accuracy at nominal temperatures. 
         [0091]    The sequence described is only one specific representation of the method of using different window sizes to extract saturation and electrical conductivity. The method applies more generally to using different window sizes. 
         [0092]    For example a single capacitor may be successively applied to the probe terminals and the probe terminals being successively shorted together while the capacitor is placed across the integrator configured to a reference voltage. Likewise, a pair of capacitors may be successively applied to the probe and an integrator. In each case at least two different measurements are made using different window sizes. 
         [0093]    Regarding wireless soil profile measurements being controlled by a Radio System on Integrated Chip (CPU). The Radio CPU wakes up periodically such as four (4) times a day and performs the tasks of making measurements from the various sensors typically by issuing commands via an I2C bus. The Radio CPU also provides the necessary functions for making a moisture measurement. Once it has collected the soil profile data including temperature, humidity, light level, saturation, and electrical conductivity it powers up the radio module and sends the data via LoRaWan™, SigFox™, WiFi or other modulation scheme to an Internet based big data cloud service or private database. The service combines the data with other sensor data and satellite and forecast data and applies heuristics to the data and issues updates and demands for irrigation. 
       EXAMPLE 
       [0094]    Measurements were taken on soil samples of white clay with various amounts of water content using the example values presented above. 
         [0095]    Saturation is defined as the ratio of volume of water to the total volume of space or voids in the soil. The samples were independently measured using density of water 1 g/cc and the density of air dried white clay 0.97 g/cc to have the Saturations of 7%, 25%, 37%, 38%, 55%, 72%, and 89%. 
         [0096]    Examination of  FIG. 6  shows the ADC measurement values of each of the samples electrical models using two different window sizes; M1 of 0.55 microseconds and M2 of 2.0 microseconds. The measurements were taken with a duration of 2 milliseconds using a 20 Volt peak to peak stimulus with period of 4.4 microseconds. 
         [0097]    Extraction of Saturation was accomplished using the formula Saturation=2M1−M2. The calibration measurement of zone “None” is used to offset parasitic capacitance of the apparatus. Zone “Ref” is used to map the Saturation to Percent Volumetric Water Content. This formula is based generally on the windows sizes of 0.55 microseconds and 2.0 microseconds. The equation used may be adapted to address specific window sizes and other patristics of the apparatus or environment as necessary such as when in containers or varying temperature and the like. 
         [0098]      FIG. 7  shows the Measured Saturation vs the Gravimetric Computed Saturation in this example. The results agree within 2% over the entire range of very dry to completely saturated white clay. 
         [0099]    Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.