Abstract:
A soil moisture sensor uses a non-collimated light source and a photosensor, respectively, mounted at the foci of a transparent ellipsoidal plastic body. The dimensions of the body are such that emitted light rays are internally reflected toward the photosensor at the surface of the ellipsoid if the surface is dry, but refracted outwardly of the body when the surface is wet. The amount of light reflected onto the photosensor is thus a measure of the amount of moisture at the surface of the sensor. Direct illumination of the photosensor by the light source is prevented either by interposing opaque electronic components between them on a circuit board, or by taking advantage of light source characteristics to minimize the amount of transmitted light. If a circuit board is used, it is completely encapsulated against moisture penetration by fixing it in a carrier and molding the body around and onto the carrier to form a monolithic unit with the carrier and circuit board.

Description:
RELATED APPLICATIONS 
   This application claims priority to U.S. Provisional Application Ser. No. 60/605,178 filed Aug. 27, 2004 entitled Optical Moisture Sensor and Method of Making the Same and is hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   This invention relates to optical moisture sensors for irrigation systems, and more particularly to a soil moisture sensor using a solid, transparent ellipsoidal body with a non-collimated light source embedded at one of its foci to reflect light toward a photosensor embedded at the other focus of the body if the soil contacting the surface of the body is dry, or to refract it outwardly of the body if the soil is wet. 
   BACKGROUND OF THE INVENTION 
   Optical sensors for determining the moisture content of the soil in an irrigation system are well known. They usually take the form of a prism or similar structure, in which a light beam projected into the prism is internally reflected toward a photosensor such as a photodiode or phototransistor. (The term “light” in this application is meant to include infrared radiation). The amount of light received by the photosensor depends on the amount of moisture present at the surfaces of the prism. This moisture changes the optical characteristics of the prism surface and thereby causes a portion of the beam to be refracted outwardly of the prism, instead of being reflected inwardly toward the light sensor. The amount of refraction, and thus the amount of light received by the photosensor, translates into a measurement of the wetness of the soil. 
   It has previously been proposed in Benoit et al. U.S. Pat. No. 4,422,714 to use a transparent half-ellipsoid body as a level sensor in a container of mineral oil. In that patent, a fiber-optic light guide conveying substantially collimated light from a light source to the ellipsoid&#39;s surface is terminated at one of the foci of the ellipsoid, while a second light guide conveying light to a photosensor receives similarly collimated reflected light at the other focus of the ellipsoid. If all or part of the convex surface of Benoit&#39;s body is immersed in mineral oil, the resulting change in the index of refraction at the body-oil interface causes the light received by the photosensor to indicate not only the presence of a critical oil level but also whether it is rising or falling. 
   The above-described prior art construction is not, however, practical for soil moisture sensors because the presence of particulates in soil requires using the maximum available surface area of the ellipsoidal body as a reflection surface, so as to average the moisture effects over as large a surface of the sensor body as possible. This in turn requires a wide-angle light source and a wide-angle photosensor at the foci of the ellipsoid. One solution to this problem is shown in my copending application Ser. No. 11/214,100, filed on Aug. 29, 2005 and entitled Optical Moisture Sensor the contents of which are hereby incorporated by reference. That application discloses a cylindrical sensor with an interior refracting surface that causes divergent light rays to be refracted into parallelism so as to make optimum use of the cylindrical soil-contacting surface of the sensor. 
   A disadvantage of the sensor shown in the above-cited copending application in cold and moist environments is the fact that an air space needs to exist between the light source or photosensor and the internal refracting surface. In a cold environment, condensation can occur in that air space, and in a very moist environment, moisture can migrate through the sensor material. In either event, these conditions may adversely affect the parallelism of the internally refracted rays and may require special manufacturing precautions. 
   The aforesaid disadvantage can be overcome by mounting a wide-angle light source and photosensor in direct contact with a transparent ellipsoidal body. This does, however, cause several other problems. For one, a substantial portion of the light travels directly through the transparent body from the light source to the photosensor without being reflected by any body-air or body-water interface. Consequently, the sensitivity of such a sensor is substantially compromised. 
   Another problem arises in the manufacture of moisture sensors of the type described due to the fact that the light source and photosensor must be maintained in exact alignment with the foci of the ellipsoid during manufacture. This is necessary in order to produce consistent readings among mass-produced sensors. Also, the difference in coefficients of expansion between the body material and the circuit board on which the sensor&#39;s optical and electronic components are typically mounted can cause minute cracks adjacent the board into which moisture can migrate. It is therefore necessary to so encapsulate the light source, photosensor and associated electronics in the ellipsoidal body that moisture cannot cause any discontinuities between them and the body. 
   SUMMARY OF THE INVENTION 
   The invention solves the first problem mentioned above by mounting some of the non-light related circuit components (e.g. resistors, capacitors and chips) of the moisture-sensing electronics on the circuit board between the light source and photosensor so that they prevent any non-reflected light from reaching the photosensor. 
   The invention solves the second problem by providing a plastic carrier that firmly secures and aligns the circuit board with respect to the mold in which the transparent ellipsoidal body of the sensor is formed, yet allows the body material to completely surround the board without any air interface in the light path, and to form a moisture-tight bond with the carrier in the finished unit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1   a  and  1   b  are schematic vertical sections along the axis of the inventive ellipsoidal moisture sensor illustrating the optical functioning of the sensor in dry and wet soils, respectively; 
       FIG. 1   c  is a diagram illustrating the critical angles at the surface of the ellipsoidal body of the sensor of  FIGS. 1   a  and  1   b;    
       FIGS. 2   a - c  are top plan, end elevation and side elevation views, respectively, of the sensor encapsulated with its carrier; 
       FIG. 3  is a perspective view of the finished sensor; 
       FIG. 4  is a schematic axial section of a typical spherical-nose IRED; 
       FIGS. 5   a  and  5   b  are polar and Cartesian representations, respectively, of the light energy distribution in an alternative embodiment of the invention; 
       FIGS. 6   a - d  are plan, side, end and schematic sectional views, respectively, of the alternative embodiment; 
       FIG. 7  is a perspective view of the alternative embodiment; and 
       FIG. 8  is an electrical diagram of a preferred circuitry for the inventive sensor. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1   a  and  1   b  illustrate the functioning of the invention. The sensor  10  consists of a circuit board  12  carrying a light source such as, e.g., an infrared emitting diode (IRED)  14 , a light sensing device such as, e.g., a photosensor  16 , and a component package  18 . The components of the package  18  may, for example, include transformers, capacitors and/or resistors, or other components suitable for causing the IRED  14  to produce appropriate illumination and to cause the illumination received by the photosensor  16  to be translated into usable signals. In accordance with the invention, the package  18  is positioned on the circuit board  12  between the IRED  14  and the photosensor  16 . The package  18  is opaque and taller than the elevation of the IRED  14  and photosensor  16  with respect to the circuit board  12 , so as to shade the photosensor  16  from direct illumination by the IRED  14 . 
   An ellipsoidal body  24  of a transparent plastic such as cyclic olefin copolymer (COC) or acrylic polymer is formed over, and in intimate contact with, the circuit board  12  and the IRED  14 , photosensor  16  and package  18  positioned thereon. The IRED  14  and the photosensor  16  are wide-angle devices and are positioned, respectively, at the two foci  26  and  28  of the ellipsoidal body  24 . Therefore, any rays emitted by the IRED  14  between the limit rays  30   a  and  30   n  are reflected at the ellipsoidal surface  32  of the body  24  toward the photosensor  16  as long as they impinge upon the surface  32  at an angle greater than the critical angle C dry  ( FIG. 1   c ) if the surface  32  is dry, or the critical angle C wet  if the surface  32  is wet. For acrylic as the body material, C dry  is 42.16°, and C wet  is 63.20°. 
   Thus, the dimensions of the ellipsoidal body  24  must be such that all rays  30  between the limit rays  30   a  and  30   n  impinge upon the ellipsoidal surface  32  at an angle P between about 43° and 63°, which is greater than C dry  but smaller than C wet  (the angle P is smallest for the rays  30   a  and  30   n , and largest half way between them). An examination of  FIGS. 1   a  and  1   b  will show that the angle P in the example described varies between 45° and about 52°. All rays  30  between  30   a  and  30   n  are thus reflected toward the photosensor  16  when the sensor surface  32  is dry ( FIG. 1   a ), but are refracted outwardly of the body  24  when the surface  32  is wet ( FIG. 1   b ). As the water content of the soil increases as a result of irrigation, more and more of the surface becomes wetted and thus governed by C wet . Since P is less than C wet , the rays striking these portions of the surface will be refracted away, reducing the number of rays traveling to the photo detector. The signal from the photosensor is proportional to the amount of rays hitting it, so it becomes also proportional to the amount of surface that is not wetted. For most types of soil, the surface of the sensor will become wetted in a piecewise continuous manner with respect to the amount of moisture in the soil. Because the sensor  10  is normally embedded in soil whose particulates attract moisture away from the surface in some proportion to the lack of moisture content in the soil, the change from reflection to refraction is not sudden but gradual with increasing moisture content of the soil. Consequently, the amount of illumination received by the photosensor  16  is a measure of soil moisture. 
   It will be understood that inasmuch as  FIGS. 1   a  and  1   b  are axial vertical sections of the sensor  10 , the rays  30  are actually half cones whose tips are at the foci  26  and  28 , and whose axes are parallel to the axis A of the sensor  10 . Consequently, the active or usable surface of the sensor  10  is the entire ellipsoidal surface  32  lying between the limit rays  30   a  and  30   n  which form the minimum practical angle (about 45°) with the surface  32 . To facilitate manufacturing by injection molding, tapered cylindrical extensions  35  are provided on each end of the ellipsoidal portion. All rays other than those between rays  30   a  and  30   n  are reflected or refracted away from the photosensor  16 . 
   Because humidity can over time migrate through plastic into any air gaps that may be in the light path, and because such humidity is likely to produce light-scattering beads of condensate, it is important that there be no air gap or air interface between the IRED  14  and the body  24  or between the photosensor  16  and the body  24 . In order to prevent such an air gap, and in order to hold the IRED  14  and photosensor  16  in exact alignment with the body  24 , the circuit board  12  of the inventive sensor  10  is entirely encapsulated within the body  24  by injection molding or another suitable manufacturing process. This is accomplished by tightly fitting the circuit board  12  into a two-piece carrier  34  (best seen in  FIG. 3 ) which, when inserted into the mold cavity of a molding machine (not shown), prevents any movement of the circuit board  12  during the preferred injection molding process. The carrier  34  is equipped with spaced ribs  36  whose interstices allow the plastic material of the body  24  to flow freely around it during the molding process. The ribs  36  also serve to hold the carrier  34  in the molding cavity so that it cannot move during the molding operation. 
   In addition, care must be taken in the molding process to avoid the formation of bubbles in the area used by the light rays  30 , and to make sure that the body material thoroughly “wets” the IRED  14  and photosensor  16  without any air between them, for the same reason as discussed above. 
   The material of the carrier  34  is preferably of a type that bonds with the material of the body  24  so as to form a tight seal with it during the molding of the body  24 . The complete encapsulation of the circuit board  12  and carrier  34  also prevents any migration of moisture into the electronics if minute cracks form in the circuit board  12  due to the difference in coefficients of expansion between the circuit board material and the material of the body  24 . 
   The molding process incorporates the circuit board  12 , body  24  and carrier  34  into a monolithic sensor unit  10  shown in  FIGS. 2   a - c  and  3  (the electrical wires interfacing the sensor  10  with external circuitry in an irrigation system are encapsulated with the circuit board  12  and are schematically shown as a cable  40  in  FIGS. 2   a - c ; they are not shown in  FIG. 3 ). The completed unit  10  is then usable as is without further processing. It will be understood that the circuit board  12 , its solder connections and electronic components must be sufficiently heat-resistant to withstand the high temperatures encountered in injection molding. 
   An alternative embodiment  48  of the invention is illustrated in  FIGS. 4 through 8 . In that embodiment, the half-ellipsoidal body  24  is replaced with a full-ellipsoidal body  50 . There is no circuit board  12 , and the IRED light source  52  and photosensor  54  are mounted directly on a carrier  56  which is then encapsulated in the body  50  by injection molding or other appropriate process. The IRED light source  52  is of a type that has a current limiting resistor built into its housing, and both the IRED  52  and the photosensor  54  are equipped with a spherical lens  58 . IREDs and photosensors of that construction are standard items in the industry, and are widely commercially available. 
   The embodiment  48  has several advantages over the embodiment  10  described above. First, the usable reflective surface of the sensor in embodiment  48  is about triple that of embodiment  10  for a given sensor size, thereby making the sensor  48  much more accurate and reliable. Secondly, the absence of a circuit board eliminates the need for caution in the molding process to avoid formation of moisture-attracting cracks in the circuit board  12  as discussed above, while at the same time reducing manufacturing costs. Also, the absence of a circuit board and the incorporation of the current-limiting resistor in the IRED assembly eliminates heat-sensitive solder joints. The IRED and photosensor assemblies have enough thermal mass to protect them against the brief thermal spike that occurs during the injection molding process. 
   Thirdly, as discussed in more detail below, the full encapsulation of the spherical lenses  58  dramatically reduces the direct, unreflected transmission of light from the IRED  52  to the photosensor  54 , to the point where interposition of an opaque component between the IRED  52  and the photosensor  54  becomes unnecessary. Fourthly, the use of a current loop, discussed below, for conveying the output of the sensor to the electronics which use its signal, improves the sensor&#39;s resistance to noise and reduce its cost. 
   The essentially total elimination of direct light transfer from the IRED  52  to the photosensor  54  without any intervening light barrier, in accordance with the invention, takes advantage of the characteristic energy distribution of spherical-lens IREDs. In this type of IRED, the light source is a chip  60  ( FIG. 4 ) that emits light mostly at about a 45 degree angle to the axis  62  of housing  64 . The glass lens  66  is shaped to focus this divergent light (as e.g. at  65 ) toward the axis  62  when the IRED  52  is in air. 
   Thus, in the intensity distribution diagram of  FIG. 5   a , curve  60  shows that the maximum light energy is emitted axially of the IRED assembly, and tapers off to zero in the direction transverse to the axis  62 , when the IRED  52  is in air. When the IRED&#39;s glass lens is encapsulated, however, in a transparent material with a refractive index similar or equal to that of glass, such as COC or acrylic polymer, the focusing effect of the lens  66  is nullified, and the intensity distribution of the IRED&#39;s light becomes substantially that of curve  68 . Curve  68  shows that in the encapsulation of the invention, the light intensity emitted by the IRED  52  thus peaks at about 45 degrees from the axis and is minimal in the axial and transverse directions. 
   As a practical matter, only light emitted at angles of about 10 degrees to 80 degrees from the axis  62  will usefully strike the surface of the body  50  and be reflected (if the surface is dry) toward the photosensor  54 . Thus, the energy useful for moisture measurement in the full ellipsoid of embodiment  48  is that emitted between lines  70   a  and  70   b , and between lines  72   a  and  72   b , in  FIG. 5   a . The total quantity of useful light is a function of the toroid whose axial cross section is the area bounded by lines  70   a ,  70   b  and curve  68 , and by lines  72   a ,  72   b  and curve  68 . 
   Light emitted by the IRED  52  in a cone of about 3 degrees on each side of the axis  62 , i.e. between lines  71  and  73 , will strike the photosensor  54  directly. Not only is that cone very small, but the light energy within that cone, as shown by curve  68 , is minimal. Mathematically, because the plot of  FIG. 5   a  is in two-dimensional polar coordinates, the angles need to be plotted along a Cartesian axis, and the intensity needs to be multiplied by the sine of the angle to accurately represent the three-dimensional reality of the situation. This is shown in  FIG. 5   b.    
   As can be seen in  FIG. 5   b , the ratio of direct light to reflectable light is quite dramatic. By integrating the curve  68  between lines  71  and  73 , and dividing the result by the sum of the integrals of curve  68  between lines  70   a  and  70   b , and between lines  72   a  and  72   b , the ratio can be calculated to be approximately 0.0009. This minute error (less than 0.1%) caused by the direct illumination of the photosensor  54  is negligible for all practical purposes. 
     FIGS. 6   a - d  illustrate the ellipsoidal body  50  encapsulating the carrier  56 . Shown in those figures, but better visible in  FIG. 7 , are the two-wire connections that connect the IRED  52  and photosensor  54  in parallel. As shown in  FIG. 8 , the power supply  74  for the sensor  48  is equipped with a current sensor  76  which produces the output signal of sensor  48 . The current-limiting resistor  78  maintains the current drawn by the IRED  52  at a constant level, while the amount of current drawn by the photosensor  54  varies in accordance with the amount of moisture present at the surface of the body  50 . By connecting the IRED  52  and photosensor  54  in parallel, a current loop is formed which requires just two wires  80 ,  82  in the sensor  48 , instead of the conventional three or four, for cost savings and easier manufacture. Also, it has been found that the current loop approach of  FIG. 8  substantially reduces the sensitivity of the sensor  48  to extraneously induced electrical noise, which has been known to be a problem in irrigation installations. 
   It will be understood that the above-described embodiments are only representative of the invention, and that its scope is to be limited only by the appended claims.