Microwave Window And Antenna Apparatus For Moisture Measurement Of Fluidized Material

Abstract

A microwave window and antenna apparatus is described for measuring the moisture content of "fluidized" material including particulate material, such as sand, or liquid material, such as oil. The container for the fluidized material is provided with a pair of microwave windows which project into the container and are in alignment with microwave transmitting and receiving antennas that cause a microwave beam to be radiated through such windows and the fluidized material and partially absorbed by the moisture therein. The microwave windows include flat inner end portions which extend parallel to each other and perpendicular to the microwave beam axis. The microwave antennas are in the form of dielectric rods and are each surrounded by a sheath of dielectric material having a dielectric constant lower than that of the antenna rod and higher than air which fills the space between such rod and the window. This prevents undesirable reflection and refraction of the microwaves and transmission of such microwaves as surface waves along the container surface. When the fluidized material is sand or other abrasive material, the microwave window is made of a hard polycrystalline ceramic, such as aluminum oxide, to reduce wear.

Description

BACKGROUND OF THE INVENTION

The subject matter of the present invention relates generally to microwave apparatus for measuring the moisture content of "fluidized" material including particulate material, such as sand, as well as liquids, such as oil. In particular the microwave moisture measurement apparatus of the present invention includes pairs of improved microwave windows covering openings in the opposite sides of the container holding the fluidized material to enable a microwave beam to be radiated through such material. The microwave windows project into the container and include flat inner end portions which extend substantially parallel to each other and perpendicular to the microwave beam axis in order to eliminate refraction of the microwave beam at the window and to prevent the microwaves from being transmitted as a surface wave over the inner surface of the container. Microwave transmitting and receiving antennas in the form of dielectric rods are positioned outside the microwave windows and are each surrounded by a sheath of dielectric material of lower dielectric constant which fills the space between the rod and window. The dielectric windows are made of a thickness equal to one-half the wavelength of the microwaves to prevent reflections from the window surfaces from causing standing wave patterns. In addition, the microwave beam axis can be provided at an acute angle between 40.degree. and 65.degree. to the container wall in order to prevent microwave reflections back to the transmitting antenna which tend to produce standing wave patterns that cause inaccuracies in the moisture measurement, as discussed in my earlier U.S. Pat. No. 3,534,260.

It is, therefore, one object of the present invention to provide an improved microwave moisture measurement apparatus for measuring the moisture content of fluidized material.

A further object of the present invention is to provide such a microwave moisture measurement apparatus of high accuracy and sensitivity in which the microwave beam is radiated through the fluidized material in a container, while preventing such microwaves from being transmitted as surface waves along the surface of the container and preventing refraction of the microwave beam as it passes through microwave windows provided on opposite sides of the container.

Another object of the invention is to provide such a microwave moisture measurement apparatus in which a pair of microwave windows are provided over openings in the sides of a container containing the fluidized material, such windows extending into the container and having substantially flat inner end portions extending parallel to each other and perpendicular to the microwave beam axis.

Still another object of the invention is to provide such a microwave moisture measurement apparatus in which the microwave transmitting and receiving antennas are in the form of dielectric rods, and the space between such antennas and the microwave windows is filled by a dielectric sheath of lower dielectric constant than such antennas.

An additional object of the invention is to prevent microwave reflections back to the antennas which produce standing wave patterns that tend to cause inaccurate moisture measurements, by providing the microwave beam axis at an acute angle with respect to the container wall and by providing the microwave window with a thickness equal to one-half the wavelength of the microwaves.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will be apparent from the following detailed description of certain preferred embodiments thereof and from the attached drawings of which:

FIG. 1 is a perspective view of a microwave moisture measurement apparatus in accordance with the invention suitable for measuring particulate material within a hopper;

FIG. 2 is an enlarged vertical section view taken generally along the line 2--2 of FIG. 1;

FIG. 3 is an enlarged vertical section view of the microwave window and dielectric rod antenna assembly employed in the apparatus of FIGS. 1 and 2;

FIG. 4 is a plan view of another embodiment of the present microwave moisture measurement apparatus suitable for measuring liquid in a pipe, with parts broken away for clarity;

FIG. 5 is a horizontal section view taken along the line 5--5 of section 4;

FIG. 6 is a plan view of a third embodiment of such microwave moisture measurement apparatus for measuring liquid in a pipe, with parts broken away for clarity; and

FIG. 7 is a horizontal section view taken along the line 7--7 of FIG. 6.

DESCRIPTION OF PREFERRED EMBODIMENT

As shown in FIGS. 1 and 2, one embodiment of the present invention includes a container 10 which may be of the hopper type containing moist particulate material such as sand. The hopper is provided with a pair of microwave windows 14 and 16 covering openings on opposite sides of such hopper which enable a microwave beam to be transmitted through such particulate material from a transmitting antenna 18 to a receiving antenna 20 mounted outside of such windows. The microwave signals are produced in a conventional manner by a microwave generator 22 and transmitted to the antennas 18 which each radiate a microwave beam of a frequency absorbed by water which has a resonant frequency of about 22 gigahertz. Thus, a portion of the microwave beam is absorbed by the water in the moist particulate material 12, while the remainder of such beam is transmitted to the receiving antenna 20. The microwave signal outputs of the receiving antennas are connected to a suitable microwave detector circuit 24 that produces an output signal voltage proportional to the percentage moisture content of the particulate material, as discussed in my earlier U.S. Pat. No. 3,534,260.

The microwave windows 14 and 16 are mounted on inwardly sloping metal sidewalls 26 and 28 of the hopper which extend downward to the discharge chute. The axis 30 of the microwave beam forms an acute angle in the range of between 40.degree. and 65.degree., and preferably about 53.degree., with the container walls 26 and 28 in order to prevent microwave reflections from such side walls from reaching the transmitting antenna 18 and producing standing waves which tend to cause inaccuracies in the moisture measurement. As shown in FIG. 1, two pairs of transmitting and receiving antennas 18 and 20 may be employed to cover the entire width of the hopper, and the microwave detector circuit 24 is connected to give a moisture percentage reading which is the average of these two microwave beams.

The microwave windows 14 and 16 each project into the container and are provided with a relatively flat inner end portion 32 which extends substantially parallel to the other inner end portion of the opposite window and substantially perpendicular to the axis 30 of the microwave beam. This prevents refraction of the microwave beam when it passes through the window and also prevents the microwaves from being transmitted as surface waves along the container surface which tend to give inaccurate moisture measurements. In addition, the microwave windows 14 and 16 are provided with inner end portions 32 of substantially uniform thickness equal to one-half the wavelength of the microwaves so that microwave reflections from the inner surface and the outer surface of such window cancel each other and thereby prevent standing wave patterns from being produced.

While the microwave windows may be made of any suitable dielectric material which does not absorb microwaves, it has been found that a hard crystalline ceramic material, such as an aluminum oxide ceramic, is preferable when measuring sand or other abrasive particulate material. The bottom of the hopper is normally closed by a discharge chute including two center opening doors 34 when moisture measurements are being made. It should be noted that the hopper doors are mounted on pivots 35 and 36 and are pivoted to open and close the hopper. However, when the doors are opened to discharge sand from the hopper, such sand runs down along the surface of the microwave windows and would abrade such windows if they are not of a harder material than such sand. Of course, aluminum oxide ceramic or "alumina" is much harder than sand since alumina has a hardness of 9 on the original mhos scale, while sand has a hardness on the order of 6 or 7 mhos, depending upon whether it contains quartz or other similar hard material.

As shown in FIG. 3, the microwave antennas 18 and 20 are both in the form of dielectric rods of a tapered conical shape and may be made of any suitable material of a high dielectric constant, such as aluminum oxide ceramic. While conventional microwave horns can be employed as the antennas, the dielectric rod antennas have the advantage that they are of smaller size and are more easily imbedded in a surrounding dielectric as hereafter described. The microwave beam is radiated from the output end 38 of the transmitting antenna 18. The transmitting and receiving antennas 18 and 20 are both surrounded by a sheath 40 of dielectric material having a dielectric constant which is lower than that of the antenna and window and is higher than that of air. For example, the sheath 40 may be of polyethylene plastic material having a dielectric constant of about 2.4 which is approximately the same as the dielectric constant 2.5 to 3.0 of the dry particulate material 12, while the alumina ceramic antenna and window have a dielectric constant of 9. The dielectric sheath 40 fills the entire space between the microwave window 16 and the antenna 18 and prevents microwave reflections by reducing the dielectric constant discontinuities in the microwave beam path since the dielectrics on opposite sides of the window have substantially the same dielectric constant. A metal casing 42 is provided around each antenna and is clamped by bolts 44 to the microwave window 16 which acts as a cover to close the open end of such metal casing and thereby forms the complete antenna housing. A layer of microwave absorber material 46, such as plastic foam containing carbon particles, is provided on the inner surface of the metal casing 42 in order to absorb microwave rays reflected from the interior of the housing to reduce the reflected microwaves that escape through the microwave window. Of course, the metal casing 42 stops all microwaves from being transmitted into or out of the housing through such casing and prevents microwaves from traveling between the antennas around the outside of the hopper. The microwave signal is transmitted into the input end of the transmitting antenna 18 through a metal launching pin 48 extending perpendicular to the side of such input end.

The microwave window 16, antenna 18 and casing 42 are all mounted on the container side wall 28 by a tubular metal sleeve 50 welded at one end to the side wall and fastened at its other end by bolts 44 to the metal casing 42 and window 16. It should be noted that rubber gaskets 52 are provided between the ceramic window 16 and metal flanges 54 and 56 attached to the sleeve 50 and the metal casing 42, respectively, to hermetically seal the antenna housing to keep out moisture and to prevent cracking of the ceramic during clamping by bolts 44. In a similar manner a liner 58 of plastic, such as polyvinyl chloride filled with microwave absorbing particles may be provided between the outer surface of the ceramic window 16 and the inner surface of the metal sleeve 50 to prevent cracking of the ceramic window and to act as a microwave absorber which prevents the launching of surface microwaves into the hopper along sleeve 50.

As shown in FIGS. 4 and 5, the microwave window and antenna apparatus described above may be employed for measuring the moisture content of a liquid, such as oil, transmitted through a pipe 60. Thus, the microwave windows 14 and 16 extend into the pipe and have their inner end portions 32 substantially parallel to each other and perpendicular to the microwave beam axis 30. In this embodiment, the microwave beam axis 30 is in a direction substantially perpendicular to the pipe axis 62, but is displaced downwardly from such pipe axis to prevent microwave reflections from the pipe wall from reaching the transmitting antenna 18 and causing the standing wave patterns to be produced.

Another embodiment is shown in FIGS. 6 and 7 which is similar to that of FIGS. 4 and 5 except that the pipe 60' is provided with openings in the opposite sides thereof which are longitudinally spaced from each other so that the microwave windows 14 and 16 covering such apertures are skewed with respect to the pipe axis 64. As a result, the microwave beam axis 30 intercepts the pipe axis 62 and the pipe wall at an acute angle in the range of between 40.degree. to 65.degree., and preferably about 53.degree., in order to prevent microwave reflections from being transmitted from the inner surface of the pipe back to the transmitting antenna and causing the formation of standing wave patterns.

While the same materials can be employed to provide the microwave windows and antennas in the embodiments of FIGS. 4, 5, 6 and 7 as that used in FIGS. 1 to 3, described previously, it is also possible to use different materials. Thus, a suitable plastic, such as Teflon, can be used for the microwave windows when they do not have to be abrasion resistant. In addition, polystyrene and polyethylene plastics may also be used for the windows as well as for antenna rods. It should be noted that for smaller dielectric rod antennas which might be necessary to fit in oil pipelines, microwaves in the x-band frequency can be employed.

It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the above-described preferred embodiment of the present invention without departing from the spirit of the invention. Therefore, the scope of the present invention should only be determined by the following claims.

Claims

I claim:

1. Microwave moisture measurement apparatus, comprising:

container means for containing fluidized material whose moisture content is to be measured and having a pair of aligned openings through the container wall;

microwave transmitting means for generating a microwave beam including at least one transmitting antenna mounted at one of said openings, said transmitting means transmitting said beam through said openings and across the entire width of all the fluidized material in the container means;

microwave receiving means including at least one receiving antenna mounted at the other of said openings in alignment with said transmitting antenna so that said microwave beam is transmitted to said receiving antenna after passing through said fluidizied material, said

transmitting and receiving antennas being dielectric rods,

mounting means for mounting the antenna rods so as to prevent said rods from contacting any of the fluidized material; and

a pair of microwave window members of dielectric material attached over said openings in said container wall and positioned between said antennas, said window members having substantially flat inner end portions extending into said container which are substantially parallel to each other and substantially perpendicular to the axis of said microwave beam transmitted through said inner end portions.

2. Apparatus in accordance with claim 1 in which the container is a hopper for holding particulate material and having inwardly sloping side walls extending downward toward the bottom of the hopper, said window members being positioned over openings in the sloping side wall so that their inner end portions extend at an acute angle to said side walls.

3. Apparatus in accordance with claim 2 in which the sloping side wall forms an acute angle with the microwave beam axis so that microwave reflections from said wall do not reach the transmitting antenna to cause standing wave patterns.

4. Apparatus in accordance with claim 1 in which the antennas rods are mounted on said windows by said mounting means.

5. Apparatus in accordance with claim 1 in which the antenna rods are each surrounded by a solid dielectric sheath which has a dielectric constant lower than that of said antenna rods, higher than that of air and substantially the same as that of dry fluidized material, said sheath filling the space between said rods and said windows.

6. Apparatus in accordance with claim 2 in which said window members are made of a polycrystalline ceramic material of high hardness.

7. Apparatus in accordance with claim 2 in which the window members are made of aluminum oxide ceramic.

8. Apparatus in accordance with claim 1 in which the inner end portions of the window member have a uniform thickness equal to one-half the wave length of the microwaves in said beam.

9. Apparatus in accordance with claim 5 in which each antenna rod is mounted in a metal casing having an open end covered by said microwave window and having a layer of microwave absorber material provided on the inner surface of said casing.

10. Microwave moisture measurement apparatus comprising:

microwave means for generating a beam of microwaves and transmitting said beam through test material to determine its moisture content, said means including at least one microwave antenna in the form of a dielectric rod;

a microwave window of dielectric material;

a sheath of dielectric material surrounding the antenna rod and filling the space between said window and said rod, said sheath having a lower dielectric constant than said rod; and

an antenna housing containing said antenna rod and said sleeve, said housing being opaque to said microwaves and having an opening covered by said window for the transmission of said microwave beam through said opening.

11. Apparatus in accordance with claim 10 in which the housing is of metal and has a layer of microwave absorber material provided on its inner surface.

12. Apparatus in accordance with claim 10 in which the microwave means includes a pair of microwave antennas in the form of dielectric rods positioned on the opposite sides of said test material for transmitting said microwave beam between said pair of rod antennas, each antenna being contained in a separate antenna housing having a microwave window and being surrounded by a sheath of dielectric material between said window and said antenna.