The present invention relates to apparatus for measuring moisture content.

BACKGROUND

Microwave systems are used in some industries to measure the moisture content of materials as they are being transported on conveyor belts. As shown in Figure 1, these systems include microwave antennae 102, 104 placed above and below a region through which a conveyor belt 106 carrying the material 108 passes. The antenna 102 underneath the belt transmits a microwave signal 110 which passes through the belt 106 and the material 108 on the belt 106, and is received by the antennae 104 above the belt. Measurement electronics 112 measure the transit time of microwaves 110 travelling from the transmit antenna 102 to the receive antenna 104, and the reduction in signal strength, and these measured parameters are used to determine the moisture content of the material 108.

Although such systems are extremely useful for materials being transported on conveyor belts, it would be advantageous to be able to provide such measurements in other industrial contexts and settings.

It is desired, therefore, to provide an apparatus for measuring moisture content that alleviates one or more difficulties of the prior art, or at least provides a useful alternative thereto. SUMMARY

Some embodiments of the present invention relate to an apparatus for measuring moisture content of at least one material in a bin having a funnel portion with non-parallel sidewall portions to provide a decreasing cross-sectional area with distance towards one end of the funnel portion, the apparatus including:

non-parallel microwave transmission windows disposed in respective ones of the non-parallel sidewall portions to allow microwaves to pass through the funnel portion of the bin;

a microwave transmission component for transmitting microwaves through the non-parallel microwave transmission windows; and

a microwave receiving component for receiving the microwaves transmitted through the non-parallel microwave transmission windows;

wherein at least one of the microwave transmission component and the microwave receiving component is configured to compensate for the microwave transmission windows being non-parallel.

At least one of the microwave transmission component and the microwave receiving component may be configured to compensate for the microwave transmission windows being non-parallel and the refraction of microwaves passing through the microwave transmission windows.

In some embodiments, the microwave transmission component includes a transmit antenna and the microwave receiving component includes a receive antenna, the transmit antenna being oriented so that microwaves transmitted into the material stored in the bin through a corresponding one of the microwave transmission windows are refracted towards the receive antenna.

In some embodiments, the microwave transmission component includes a transmit antenna and the microwave receiving component includes a receive antenna, the receive antenna being oriented so that microwaves transmitted through the material stored in the bin and through a corresponding one of the microwave transmission windows are received by the receive antenna in a direction substantially normal to the receive antenna.

In some embodiments, at least one of the microwave transmission component and the microwave receiving component includes a corresponding compensation wedge disposed between the corresponding antenna and the corresponding microwave transmission window.

In some embodiments, the microwave transmission component and the microwave receiving component are located at different distances from the end of the funnel portion to allow for refraction of the microwaves entering and/or exiting the bin.

In some embodiments, the microwave transmission component and the microwave receiving component are arranged so that microwaves are transmitted from the microwave transmission component through the stored material, reflected at an inner surface of the bin, and the reflected microwaves are transmitted through the stored material to the microwave receiving component.

Also described herein is an apparatus for measuring moisture content of at least one material in a bin having a funnel portion with non-parallel sidewall portions to provide a decreasing cross-sectional area with distance towards one end of the funnel portion, the apparatus including:

microwave transmission windows for covering respective openings formed in respective ones of the non-parallel sidewall portions to allow microwaves to pass through the funnel portion of the bin, the microwave transmission windows being configured to withstand pressure exerted by the material in the bin;

a microwave transmission component for transmitting microwaves through the microwave transmission windows; and

a microwave receiving component for receiving the microwaves transmitted through the microwave transmission windows. In some embodiments, each microwave transmission window includes a wear liner for contacting the material, and a backing plate for supporting the wear liner so that the microwave transmission window can withstand the pressure exerted by the material in the bin.

In some embodiments, the apparatus includes mounting components for mounting the microwave transmission component and the microwave receiving component adjacent respective ones of the microwave transmission windows, the mounting components being adjustable to allow the orientation of at least one of the microwave transmission component and the microwave receiving component to be adjusted to compensate for the sidewall portions being non-parallel.

In some embodiments, the apparatus includes one or more compensation wedges for mounting between at least one of the microwave transmission component and the microwave receiving component to compensate for the sidewall portions being non- parallel.

Also described herein is an apparatus for measuring moisture content of at least one material in a bin having a funnel portion with non-parallel sidewall portions to provide a decreasing cross-sectional area with distance towards one end of the funnel portion, the apparatus including:

microwave transmission windows disposed in respective ones of the non-parallel sidewall portions to allow microwaves to pass through the funnel portion of the bin; a microwave transmission component for transmitting microwaves through the microwave transmission windows; and

a microwave receiving component for receiving the microwaves transmitted through the microwave transmission windows;

wherein at least one of the microwave transmission component and the microwave receiving component is configured to compensate for the refraction of microwaves passing through the microwave transmission windows. Also described herein is an apparatus for measuring moisture content of at least one material in a bin having a funnel portion with non-parallel sidewall portions to provide a decreasing cross-sectional area with distance towards one end of the funnel portion, the apparatus including:

microwave transmission windows disposed in respective ones of the non-parallel sidewall portions to allow microwaves to pass through the funnel portion of the bin; a microwave transmission component for transmitting microwaves through the microwave transmission windows; and

a microwave receiving component for receiving the microwaves transmitted through the microwave transmission windows;

wherein at least one of the microwave transmission component and the microwave receiving component is configured to compensate for the microwave transmission windows being non-parallel and/or the refraction of microwaves passing through the microwave transmission windows.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, wherein:

Figure 1 is a schematic cross-sectional end view of a prior art microwave measurement system for measuring the moisture content of materials being transported on a conveyor belt;

Figure 2 is a schematic cross-sectional side view of a portion of a loading bin with non-parallel microwave transmission windows mounted flush with the non-parallel inner sidewalls of the bin to allow microwaves to pass right through the bin and the material stored therein;

Figure 3 includes plan and cross-sectional side views of one of the microwave transmission windows of Figure 2;

Figure 4 shows the bin of Figure 2 disposed between microwave antennae mounted vertically, illustrating the refraction of microwaves passing into the stored material away from the microwave receiving antenna; Figure 5 shows the bin of Figure 2 disposed between microwave antennae oriented to compensate for the refraction of microwaves passing through the microwave transmission windows;

Figure 6 shows an embodiment similar to that shown in Figure 5, but where the transmission and receiving windows are positioned at different heights;

Figure 7 shows an embodiment where the transmitting and receiving antenna and associated windows are located at the same side of the bin, the microwave signal being reflected from the far inner surface of the bin; and

Figure 8 shows the bin of Figure 4 with compensation wedges disposed between the vertical antennae and the microwave transmission windows to compensate for the refraction of microwaves passing through the microwave transmission windows.

DETAILED DESCRIPTION

The inventors have determined that it would be useful to provide a system that can measure the moisture content of materials in dispensing or loading bins. These are large metal bins that are commonly used to store bulk materials in the mining, minerals, cement, farming and many other industries, and including a funnel portion having at least one inclined or sloping sidewall that provides a decreasing cross-sectional area (in a horizontal plane) with decreasing height. The base of the funnel portion has a shuttered opening to allow the material stored in the bin to be dispensed or discharged from the bin as required. These bins can be many metres tall and many several metres across. The lower part of the bin with the sloping sidewall(s) is typically in the shape of an inverted cone (i.e., with a single, curved sidewall) or an inverted pyramid (with planar sidewalls). However, other configurations are possible, including those in which one wall is vertical and the opposite wall is inclined to the vertical.

In order to allow microwaves to pass through the bin 202 and the material 204 stored therein, opposing portions of the sloping metal bin 202 sidewalls can be replaced with electrically non-conductive portions or 'windows' 206 that allow microwaves to pass through, as shown schematically in Figure 2. Even with such windows 206, the upper part of the bin 202 is usually too wide to allow microwaves to penetrate all the way through the material 204 stored therein. However, near the base of the bin 202, where the bin 202 and consequently the material therein 204 become narrower, the inventors have determined that it is possible to make microwave moisture measurements. In particular, this is facilitated by using relatively low microwave frequencies to provide greater penetration through most materials. For example, standard microwave moisture measurement systems such as those described above for measuring moisture in materials being transported on conveyor belts use microwaves having frequencies of 2-4 GHz; however, microwaves having lower frequencies of about 1-2 GHz are able to penetrate larger distances, up to about 1 metre.

The electrically non-conductive windows 206 are configured to be sufficiently robust to withstand the pressure exerted by the material in the bin 202. In the described embodiments, each of the windows 206 consists of a fibreglass backing plate 302 for strength, and a polyethylene or polyurethane wear liner 304 in contact with the material 204 over a rectangular region of dimensions 360mm ^χ 280mm. For example, a 7 metre high bin full of sand produces a pressure of about 10 tonnes per square metre, producing a force of about 1 tonne on each window 206. As shown in Figure 3, each window 206 includes a metal frame 301 welded around a rectangular opening in the walls of the bin 202. Threaded studs 303 around the frame 301 engage with corresponding holes in the wear liner 304 and the fibreglass backing plate 302. The wear liner 304 is thicker than the bin wall 202, and has a peripheral rebate that allows a portion of the liner 304 to fit into the hole in the bin 202 and to be flush with the inside surfaces of the bin 202 to prevent material blockages and hang ups. Because the microwave transparent windows 206 are parallel with the sloping sides of the bin 202, they are not parallel with one another.

A consequence of the fact that the microwave transmission windows 202 are not mutually parallel is that the microwaves do not measure a uniformly thick slab of the material of interest 204, but rather a truncated wedge. Consequently, microwaves passing through the upper portions of the windows 206 pass through more of the stored material 204 than microwaves passing through the lower portions of the windows 206. As will be understood by those skilled in the art, a microwave transmit antenna 102 is not a point source but transmits microwaves over most of its face or frontal area. Similarly, a microwave receive antenna 104 is sensitive to microwaves over most of its frontal area. Consequently, if the antennae 102, 104 are mounted with their front faces oriented vertically, as shown in Figure 4, then a signal transmitted from the lower part of the transmit antenna 102 will travel through less of the stored material 204 than a signal transmitted from the top of the transmit antenna 102, and consequently will have a shorter transit time and less attenuation. It will be apparent that such variations in the received signals degrade the quality of the measurement. As shown in Figure 4, another disadvantage of non- vertical windows 206 is that the direction of microwave beam propagation through the windows 206 and into and from the material 204 (which is generally desired to be horizontal in order to minimise path length through the material 204) is not normal to the surface of the windows 206 or the material 204, causing the microwave beam to be refracted away from the receive antenna 104, thereby reducing the signal strength at the receive antenna 104. Refraction also causes the receive antenna 104 to receive the signal at an angle that is not normal to the face of the receive antenna 104, thereby decreasing detection efficiency.

The described embodiments of the present invention address these difficulties. In some embodiments, both of these difficulties are overcome by tilting both antennae 102, 104 so that they face slightly downwards, as shown in Figure 5. In this configuration, the refraction of the microwaves: (i) causes the maximum signal to be directed towards the receive antenna 104, and (ii) orients trie receive antenna 104 for maximum detection efficiency. The optimum angle of tilt can be determined by adjusting the angle for maximum received signal strength. The microwave refractive index depends on the material(s) in the bin, so is determined in situ for the particular material to be analysed. To facilitate this adjustment, the system includes mounting hardware that allows the antenna angle to be adjusted when the system is installed or after installation. This allows the best angle to be experimentally determined for the particular material being analysed. Selecting the tilt angle for maximum signal strength also solves the problem of variable transit time. The tilt results in the lower part of each antenna 102, 104 being further away from the window 206 than its upper part. The additional path length exactly compensates for the shorter path length through the lower part of the material due to the direct relationship between the refractive index and the velocity of microwaves through the material. Microwave refractive index is defined as the velocity of microwaves in a vacuum divided by the velocity of microwaves in the material.

In an alternative embodiment, as shown in Figure 6, rather than orient the transmitting antenna away from the vertical, the receiving antenna 104 and its associated window are positioned at a different height to receive the refracted microwave beam. A consequence of this arrangement is that the path length through the stored material 204 is greater than for the embodiment of Figure 5. However, this can be advantageous if space around the bin is restricted, and or if the additional path length increases the measurement accuracy.

In another alternative embodiment, as shown in Figure 7, the transmitting and receiving antennae and their associated windows are positioned on the same side of the bin, the microwaves being reflected from the inner surface of the opposite side or surface of the bin. A disadvantage of this embodiment is that the path length through the stored material 204 is more than double the path length for the embodiment of Figure 5. However, this arrangement may be advantageous if only one side of the bin is accessible.

In yet other embodiments, wedge-shaped compensation objects are disposed between the antennae 102, 104 and the microwave transmission windows 206 to compensate for the different path lengths through the material 204 stored in the bin 202. The wedge material is selected to have the same (or substantially the same) refractive index as the material being analysed. For example, the wedge could be a wedge-shaped fibreglass container filled with the same material as the material being analysed. Alternatively, the wedge can be made from a different material with a suitable refractive index, such as polyethylene, PTFE, or another similar material. The wedge is thicker at the bottom and introduces a delay that exactly compensates for the reduced thickness of analysed material towards the bottom of the microwave transmission windows 206. It will be apparent that the wedges could alternatively be attached to the microwave transmission windows 206 or even formed integrally therewith. However, such an arrangement would limit the flexibility of the system.

In general, a combination of wedge angle and antenna tilt can be used to optimise the received signal strength. Figure 8 shows two faces of each wedge as being respectively vertical and parallel to the wall of the bin 202, but other angles can be used for both faces to provide a desired wedge angle and desired bending of the beam.

In some embodiments, antenna orientation and wedge compensation can be used in combination, for example, in situations where the refractive index of the wedge material does not exactly match the refractive index of the material being analysed. Although the embodiments described above include a bin with one or more sloped sidewalls so that both microwave transmission windows are also inclined, it is possible that a bin might be configured with only one of two opposing planar sidewalls inclined to the vertical, the other of these sidewalls being vertical. In this case, only the antenna at the one inclined sidewall would need to be directed downwards, or alternatively only that antenna would require a compensating wedge.

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.