WO1986004151A1

WO1986004151A1 - Soil moisture monitor

Info

Publication number: WO1986004151A1
Application number: PCT/US1986/000007
Authority: WO
Inventors: Richard L. Bireley
Original assignee: Bireley Richard L
Priority date: 1985-01-02
Filing date: 1986-01-02
Publication date: 1986-07-17
Also published as: EP0205599A1; AU5302186A; AU607604B2; AU3318589A

Abstract

An electronic sensor (1) which measures the moisture level of a surrounding porous compound, such as soil, by summing the conductances of a plurality of electric paths (12-16) formed by traces of impure water, between a first exposed electrode (4) and a second electrode covered by a hydrophobic dielectric coating. The electrodes are mounted in the same plane (32, 34) parallel planes, or in non-parallel planes structured to reduce reactance between the two electrodes. Beads of water (17-21) form at the points where said traces contact the dielectric-coated-electrode, said beads functioning as point capacitors having said second electrode as a common plate. Both electrodes comprise the reactive element of an astable multivibrator (22) whose frequency varies proportionally to the number of electric paths formed by said traces of impure water. The relatively large reactive component introduced by the series-linked point capacitors tends to mask the variations in conductance between the various paths and contributes to a very linear relationship between the moisture level and the multivibrator frequency over a broad range. The output of the multivibrator is fed to a series of monostables (26-29) to generate signals corresponding to predetermined thresholds of moisture level.

Description

SOIL MOISTURE MONITOR FIELD OF THE INVENTION

This invention relates to electronic fluid and moisture level monitors, and more particularly, to variable-reactance sensors.

BAC KGROUND OF THE INVENTION

Apparatuses for detecting and measuring the presence and level of fluid in a given medium are commonly used for monitoring and controlling fluid levels, flow, relative moisture contents, liquid spills, contamination and the like.

Contemporary sensor technology depends on changes in the dielectric property of the medium caused by variances in fluid level or fluid concentration within the medium. Sensors of contemporary design typically utilize a parallel plate capacitor immersed or imbedded in the medium so that a portion of the medium may function as a dielectric between the plates. The capacitor is part of the RC circuit of an oscillator whose frequency varies with changes in the dielectric property of that portion of the medium between the plates. This technique is generally satisfactory when the dielectric-forming medium is a fluid: the gap between the plates can be narrow enough to yield a relatively high capacitance. However, when dealing with granular or pulverous media, a wider plate gap is necessary; capacitance is thus reduced to an impractically low value. Furthermore, as the fluid absorbing medium becomes increasingly saturated, the relationship between the dielectric value and fluid concentration becomes less linear. As a consequence, accuracy of fluid concentration measurement suffers. By accurately monitoring moisture levels in cultivated land in order to provide optimum irrigation, significant gains can be achieved in water and energy conservation. The control of sprinklers, pumps and other irrigation equipment from moisture monitors imbedded at plant root-level has almost become a necessity in the water-poor sun-belt western states.

Conventional moisture sensors of the type described above are not only inaccurate under certain conditions, but are difficult to imbed in the ground due to their fragile plate configuration. There is a need for a more sensitive type of soil moisture monitor packaged in an easily-imbedded, durable, compact enclosure.

SUMMARY OF THE INVENTION The principal objective of the invention is to provide a highly-accurate, linearly-sensitive moisture sensor for the control or irrigation equipment.

The secondary objective of the invention is to achieve a design which can be packaged in a rugged, compact enclosure which can be easily imbedded to plant root-level.

Both objectives are achieved by means of a simple electronic circuit packaged in a preferably cylindrical enclosure. The wall and base of the enclosure form two electrodes which sense the conductive paths, created by traces of water laden with dissolved impurities, from one electrode through the surrounding soil to the other electrode. One of the electrodes is exposed to the soil; the other is covered with a thin layer of hydrophobic dielectric coating. A bead or droplet of water forms at the point of contact between each trace and the hydrophobic coating, creating a point capacitance across the dielectric coating. As the moisture level increases, so does the number of water beads or droplets in contact with the dielectric-coated electrode. A complex impedance, having a relatively high reactive component related to the sum of all droplet-related capacitances and a relatively low resistive factor related to the sum of the conductive paths created by the water laden with dissolved impurities, is thus formed between the two electrodes. This complex impedance determines the frequency of an astable multivibrator. A frequency discriminator is used to trip a solenoid driver at a predetermined level of moisture. The driver, in turn, is used to energize a solenoid-operated start/stop switch which controls the irrigation equipment. The aforementioned technique provides a highly-accurate sensor due to the high value of capacitance which results from summing the various point capacitances. A higher degree of linearity and accuracy can be maintained as the saturation level approaches one hundred percent. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a front elevational view of a soil moisture sensor according to the invention with a section of the wall cut away to show the internal configuration;

Figure 2 is a top plan view of the sensor; Figure 3 is a diagrammatical interpretation of the point of contact between a water droplet and the dielectriccoated electrode;

Figure 4 is a diagram of a first alternate electrode arrangement; Figure 5 is a diagram of a second alternate electrode arrangement; and

Figure 6 is an electrical diagram of the sensor circuitry. DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

The preferred embodiment of the invention is illustrated in Figures 1, 2, 3 and 6 of the drawing.

Referring now to Figure 1, a soil moisture sensor 1, packaged in a generally cylindrical enclosure 2, is imbedded in a porous plant-growing medium 3 preferably at root level. Exposed electrode 4 comprises the bottom of enclosure 2. Enclosure 2 is covered cylindrically by a thin coating of hydrophobic, dielectric material 5 such as glass, porcelain or other vitreous compound, a synthetic resin polymer (e.g. polytetrafluor ethylene) or a thermosetting resin (e.g. one of the polymerized epoxides). Structural integrity is imparted to the cylindrical surface of enclosure 2 by cylindrical electrode 6 which lies directly beneath thin dielectric coating 5. Insulated wires 7 and 8 respectively connect exposed electrodes 4 and cylindrical electrode 6 to printed circuit board 9. Multivibrator chip 10 and auxiliary circuitry are mounted on printed circuit beard 9. Light-emitting-diode 11, also mounted on printed circuit board 9, lights up when the solenoid driver circuit is activated. Multiple conducting paths 12, 13, 14, 15 and 16 are formed between exposed electrode 4 and cylindrical electrode 6 by traces of water containing dissolved impurities. Point capacitances are formed between the final droplets 17, 18, 19, 20 and 21 of conducting paths 12, 13, 14, 15 and 16, respectively, and the corresponding adjacent regions of cylindrical electrode 6. The complex impedance formed between electrodes 5 and 6 functions as an RC circuit which determines the oscillation frequency of an astable multivibrator 22 on chip 10. Potentiometers 23 and 24 can be adjusted to set the multivibrator frequency thresholds corresponding to the moisture levels at which the solenoid driver circuit is activated and deactivated as explained below.

Referring now to the circuit block diagram of Figure 6, the complex impedance sensor of the moisture sensor unit is shown as block 25. The complex impedance determines the vibration frequency of astable multivibrator 22. As the moisture level of the soil increases, the resulting increase in the number of point capacitances forming on the coated electrode 6 causes a drop in the oscillation frequency of astable multivibrator 22.

The pulse-train issuing from astable multivibrator 22 is fed to two monostable multivibrators 26 and 27 connected to the retriggerable mode which functions as frequency discriminators. The timing cycles of these onostable multivibrators are adjustable by means of potentiometers 23 and 24, respectively. The timing cycle of multivibrator 26 is adjusted to match the period of the astable multivibrator 22 which corresponds to the minimum level of moisture which can be tolerated, i.e. the starting point for the watering cycle. Monostable multivibrator 29 is permanently set to have a relatively long firing period. In a manner like unto that previously described for starting the watering cycle, reference monostable multivibrator 29 will be triggered and will remain in its "fired" high-level state only after the maximum moisture level has been reached.

AND gate 30 assures that the solenoid driver 31 is enabled only when the moisture level remains between the two selected thresholds. The light-emitting diode indicator 11 is provided to facilitate adjustment of potentiometers 23 and 24. Both the potentiometers and the indicator are accessible through sealable holes in the top of enclosure 2. Adjustment of the low and high moisture thresholds may be performed either in the factory or in the field. Driver 31 is designed to control solenoid-controlled valves of the type commonly used with watering equipment.

It should be understood that the moisture level sensor may be integrated in various ways with automatic watering equipment. For instance, the equipment could be started at fixed intervals by a timer and shut off only when the maximum level of mpisture is reached. Alternatively, the equipment could be started when the maximum tolerable level of moisture is reached, then stopped after a fixed interval of watering time. The circuitry heretofore described may be readily adapted to these various modes by disconnecting the appropriate leg of and gate 30.

When used in either mode of operation, the invention can result in considerable conservation of water while maintaining ground moisture within an optimum range. Figures 4 and 5 indicate alternative configuration for the construction of two electrodes. In both Figures 4 and 5, both the exposed electrode 32 and the unexposed electrode 34 lie in a common plane, with the latter electrode completely surrounded by thin dielectric coating 33. While the preferred embodiment of the invention has been discussed, other embodiments may be devised and modifications made within the spirit of the invention and within the scope of the appended claims. WHAT IS CLAIMED IS :

Claims

1. An apparatus for measuring the level of moisture in a water-permeated material which comprises an electronic circuit having a variable complex impedance consisting essentially of: a first exposed electrode; a second electrode covered by a dielectric coating; said electrodes being imbedded in said material in an arrangement so as to minimize any capacitive effect between said electrodes; and at least one electrically-conductive path formed through a portion of said material by interconnected traces of impure water between the surface of said first electrode and the surface of said dielectric coating.

2. The apparatus of Claim 1 wherein said dielectric coating is selected and applied as a thin film in order to create at least one capacitor between the point of contact of said electrically-conductive path, said point of contact acting as the f i rst plate of said capacitor, and a section of the second electrode immediately opposite said point of contact acting as the second plate of said capacitor.

3. The apparatus of Claim 2 wherein the material of said dielectric coating is selected from a group of hydrophobic compounds including synthetic resin polymers.

4. The apparatus of Claim 2 wherein the material of said dielectric coating is selected from a group of hydrophobic vitreous compounds including glass, porcelain and ceramic.

5. The apparatus of Claim 2 wherein the material of said dielectric coating is selected from a group of thermosetting resins including polymerized epoxides.

6. The apparatus of Claim 2 wherein the face of one of said electrodes is generally perpendicular to the face of the other electrode.

7. The apparatus of Claim 2 wherein the face of one of said electrodes is generally in the same plane as the face of the other electrode.

8. The apparatus of Claim 2 wherein said circuit comprises means for generating a periodic signal whose frequency varies in response to changes in said complex impedance.

9. The apparatus of Claim 4 wherein both of said electrodes generally conform to the wall of a cylinder.

10. The apparatus of Claim 4 wherein one of said electrodes generally conforms to the wall of a cylindrical enclosure and the other electrode generally conforms to the base of said enclosure.

11. The apparatus of Claim 10 wherein the capacitance factor of said complex impedance increases generally proportionally with the number of said points of contact with the dielectric coating.

12. The apparatus, of Claim 11 wherein said circuit comprises a frequency discriminator responsive to said periodic signal.

13. The apparatus of Claim 12 wherein said circuit comprises a solenoid driver controlled by the output of said frequency discriminator.