Wireless soil profile monitoring apparatus and methods

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.

Claims

1. A method for measuring volumetric water content of a sample material using a monitoring apparatus including an integrator capacitor, a plurality of switched capacitors and first and second conductors, the conductors being proximate to the sample material, the method comprising: a. discharging the integrator capacitor; b. generating a first set of control signals having a predetermined period, a first predetermined duty cycle and a predetermined number of cycles; i. asserting the first control signal including; 1. charging a first switched capacitor to a first predetermined potential; and 2. connecting a second switched capacitor in series with the first and second conductors; ii. de-asserting the first control signal including; 1. connecting the first switched capacitor in series with the first and second conductors; and 2. charging the second switched capacitor to a second predetermined potential by transferring the charge to or from the integrator capacitor; c. upon completing steps i and ii for the predetermined number of cycles, quantifying an amount of charge transferred to the integrator capacitor and recording the amount of total charge transferred to the integrator capacitor as a first quantity; d. discharging the integrator capacitor; e. generating a second set of control signals having a second predetermined duty cycle and repeating steps i and ii above; f. upon completing steps i and ii for the predetermined number of cycles at the second predetermined duty cycle, quantifying an amount of charge transferred to the integrator capacitor and recording the amount of charge transferred to the integrator capacitor as a second quantity; and g. calculating volumetric water content and electrical conductivity using as independent variables the first quantity and the second quantity which were obtained using the respective first and second predetermined duty cycles.

2. The method of claim 1 further comprising, generating a demand notification for irrigation.

3. The method of claim 1 further comprising a voltage follower connected to coax shielding or PCB layers to form a driven shield to protect the probe terminals from parasitic capacitance.

4. The method of claim 1 wherein the sample material is soil.

5. The method of claim 1 wherein the sample material is moisture sensitive polymer.

6. The method of claim 1, further comprising, communicating by wireless transmission the calculated volumetric water content and electrical conductivity output to a server or datastore.

7. The method of claim 6 wherein the wireless transmission is a private network including Bluetooth or WiFi.

8. The method of claim 6 wherein the wireless transmission is a public network including LTE, LoRaWan™ or SigFox™.

9. A method for measuring volumetric water content of a sample material using a monitoring apparatus including an integrator capacitor, a plurality of switched capacitors including first, second, third, fourth, fifth and sixth switched capacitors and first and second conductors, the conductors being proximate to the sample material, the method comprising: a. discharging the integrator capacitor; b. generating a first control signal at a first predetermined duty cycle and a second control signal at a second predetermined duty cycle, the generating comprising: i. asserting the first control signal including; 1. discharging the first capacitor; and 2. connecting the second capacitor in series with the conductors; ii. de-asserting the first control signal including; 1. connecting the first capacitor in series with the conductors; and 2. transferring a charge from the second capacitor to the integrator capacitor; iii. asserting the second control signal including; 1. adding charge to the third capacitor; 2. connecting the fourth capacitor in series with the conductors; 3. removing charge from the fifth capacitor; and 4. connecting the sixth capacitor in series with the conductors; iv. de-asserting the second control signal including; 1. connecting the third capacitor in series with the conductors; 2. removing charge from the fourth capacitor; 3. connecting the fifth capacitor in series with the conductors; and 4. adding charge to the sixth capacitor. c. completing steps i-iv for a predetermined number of cycles for the first control signal then measuring a first accumulated voltage on the integrator capacitor voltage; and d. completing steps i-iv for the predetermined number of cycles for the second control signal then measuring a second accumulated voltage on the integrator capacitor voltage; e. calculating volumetric water content and electrical conductivity using as independent variables the first accumulated voltage and the second accumulated voltage.

10. A method for measuring volumetric water content of a sample material using a monitoring apparatus including an integrator capacitor, a switched capacitor and first and second conductors, the conductors being proximate to the sample material, the method comprising: a. asserting a first control signal including; i. charging the switched capacitor to a first predetermined potential or zero by transferring charge from the switched capacitor to the integrator capacitor; and ii. charging the sample material by connecting the first conductor and the second conductor to predetermined potentials; b. de-asserting the first control signal including; i. connecting the switched capacitor in series with the first and second conductors.

Description

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, <1 MHz Gain Bandwidth. The resulting filter response however is capable of capturing frequency components >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.