Optical moisture sensor and method of making the same

• US 20060043270A1
• Filed: 08/29/2005
• Published: 03/02/2006
• Est. Priority Date: 08/27/2004
• Status: Active Grant

• Current Assignee: Toro Company (Toro)
• Original Assignee: Toro Company (Toro)
• Inventors: Zimmerman, James
• Granted Patent: US 7,247,837 B2
• Time in Patent Office: Days
• US Class Current: 250/227.250
• CPC Class Codes: A01G 25/167 Control by humidity of the soil itself or of devices simulating soil or of the atmosphere; Soil humidity sensors
• CPC Class Codes: G01N 21/552 Attenuated total reflection

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.

Claims

1. A soil moisture sensor, comprising:

a) a solid, transparent body substantially in the shape of a half ellipsoid;

b) a non-collimated light source positioned at a first focus of said half ellipsoid; and

c) a photosensor positioned at a second focus of said half-ellipsoid;

d) said light source and said photosensor being mounted on a circuit board entirely encapsulated in said body;

e) said circuit board having at least one opaque element mounted thereon between said light source and said photosensor in such a manner as to prevent unreflected light from said light source from reaching said photosensor.

2. The sensor of claim 1, in which said opaque element is a circuit element associated with said light source or said photosensor.

3. A soil moisture sensor, comprising:

a) a solid, transparent body substantially in the shape of a half ellipsoid;

b) a non-collimated light source positioned at a first focus of said half ellipsoid; and

c) a photosensor positioned at a second focus of said half-ellipsoid;

d) said light source and said photosensor being mounted on a circuit board;

e) a carrier holding said circuit board against movement with respect thereto;

f) said body being part of a monolithic unit encapsulating said light source, photosensor, circuit board and carrier.

4. A method of making an ellipsoidal soil moisture sensor, comprising the steps of:

a) providing a carrier arranged to be firmly held in a mold cavity;

b) providing a circuit board carrying a light source and a photosensor in predetermined positions;

c) suspending said circuit board in said carrier against movement with respect thereto; and

d) forming around said circuit board and carrier a monolithic, at least partially ellipsoidal, transparent body, said body encapsulating said circuit board and carrier therein in a position where said light source and photosensor are positioned at the foci of said ellipsoidal body;

e) the material of said body being such as to bond with the material of said carrier.

5. The method of claim 4, further comprising the step of providing on said circuit board an opaque element so positioned as to prevent the transmission of unreflected light from said light source to said photosensor.

6. The method of claim 4, in which said light source and photosensor are noncollimated.

7. The method of claim 4, in which said body is formed by injection molding, and the materials of said light source, circuit board, photosensor and carrier are chosen to withstand injection molding temperatures.

8. The method of claim 4, further comprising the step of preventing the formation of bubbles or air interfaces in the path of any light rays from said light source that are reflected toward said photosensor.

9. A soil moisture sensor, comprising:

a) a solid, transparent ellipsoidal body;

b) a non-collimated light source positioned at a first focus of said body;

c) a photosensor positioned at a second focus of said body;

d) light from said light source being reflectable toward said photosensor internally of said body when the surface of said body is dry but not when it is wet; and

e) said moisture sensor being arranged to minimize transmission of unreflected light directly from said light source to said photosensor.

10. The soil moisture sensor of claim 9, in which said transmission is minimized by interposing opaque elements in said body between said light source and said photosensor.

11. The soil moisture sensor of claim 9, in which said transmission is minimized by minimizing the light energy output of said light source in the direction toward said photosensor.

13. The soil moisture sensor of claim 9, in which said body is in the shape of a full ellipsoid.

14. The soil moisture sensor of claim 9, in which said light source and photosensor are connected in parallel to form a current loop.

12. The soil moisture sensor of claim 12, in which said light source has a lens which focuses its light energy along its axis when said light source is in air, but which causes said light energy to be principally emitted away from said axis when said light source is encapsulated in a material with an index of refraction similar to that of the material of said lens.