AU648585B2 - Moisture detection using a radio frequency resonator - Google Patents

Description

648585

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION (Original) Name of Applicant/Nominated Person: COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION Actual Inventors: PETER IVAN SOMLO JOHN EVAN PETERS GRAHAM RAYNOR ALLEN DESMOND CLIFTON ARTHUR FRANK MARTIN WARNER Address for Service: DAVIES COLLISON, Patent Attorneys 1 Little Collins Street, Melbourne, Victoria 3000

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Invention Titlet "MOISTURE DETECTION USING A RADIO FREQUENCY RESONATOR" Details of Associated Provisional Application: PROVISIONAL PATENT APPLICATION No PK 2602 The following statement is a full description of this invention, including the best method of performing it known to us: 1 -2- Technical Field This invention concerns monitoring of the water content of materials. More particularly, it concerns apparatus and a method for monitoring the moisture present in materials using the absorption by the materials of electromagnetic power at radio frequencies. It is particularly but not exclusively suitable for the measurement of the moisture level of sugar and other granulated materials.

Soo: 10 In this specification, the term "radio frequency" means a frequency which is in the range of vhf frequencies, uhf frequencies, and microwave frequencies. Microwave frequencies are preferred for the present invention.

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F'ackground 15 There is often a need to measure the moisture content of materials. For example, the moisture content of soil is an important parameter in plant growth. The moisture content of stored grains is a critical factor in the storage of wheat, rice, legumes, coffee and other grains and pulses, and the moisture content of radar housings (for example, those used in aircraft) must not exceed a specified value if the radar is to runction in a useful manner. A particular monitoring need that has been addressed by the present inventors is that of the moisture content of raw sugar, ind as the consideration of this need led to the present invention, it will be discussed in some detail in this S r f' c--1 nr

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3 The water content of raw sugar is critical to its handling. If the raw sugar crystals are dry, they coalesce and cease to be free running. If the raw sugar crystals have a free water content of 1 per cent or more, the sugar is too wet to handle in the conventional manner. Thus, for satisfactory handling of raw sugar crystals, the free moisture in the bulk of the crystals must be greater than zero but less than about 1 per cent.

10 Before the development of the present invention, the 4 measurement of the free water content in raw sugar crystals has been carried out at sugar mills using one of two techniques. Both techniques are still in use.

1: The more reliable of these two techniques involves 5.1.5 extraction of sugar samples from the bulk sugar and drying the samples in an oven at a predetermined temperature. The sugar crystals are weighed before and after the drying, and the free water content of the sample is obtained from the difference between the two a 20 weighings. This procedure provides an accurate value of the free water content of the sugar, but it takes a considerable time to complete. Thus this method can provide only an occasional determination of the free water content in a production line handling large volumes of raw sugar.

The alternative technique for determining the free water content of raw sugar involves the use of a "quadrabeam" analyser, the basis of which is a measurement of the rotation of the plane of polarisation of a linearly polarised light beam. The 4 quadrabeam analyser is effective for the determination of the free water content of raw sugar produced from cane from the same farm, but the instrument requires a separate calibration for each sugar source. This is understood to be because the reading obtained with a quadrabeam analyser is sensitive to the "dilution indicator" of the sugar (the dilution indicator being the ratio of the free water content of the bulk of crystals to the impurity content of the sugar, both 10 values being expressed as a percentage of the mass of the sugar). Quadrabeam analysers, which are expensive items of equipment, are thus of limited use in sugar mills that receive sugar cane from different farms (which includes all large sugar mills).

Disclosure of the Present Invention It is a prime objective of the present invention to provide a method, and equipment that is relatively inexpensive to construct, with which a quick and accurate indication of the water content of samples of raw sugar can be obtained. (As noted above, the *I invention is not limited to this application.) This objective is achieved by putting a sugar sample in a radio frequency resonator and obtaining an indication of the Q-factor of the resonator. The resonator is provided with two probes, each at an electric or magnetic antinode position in the resonator cavity, to enable the strength of the signal in the resonator to be monitored. The probes one an input probe which couples a radio frequency signal to the cavity, one an output probe which monitors the resonant signal in the 5 cavity may be located at substantially the same antinode position, or at different antinode positions.

The probes may both be capacitive probes, or they may both be inductive probes, or one probe may be a capacitive probe and the other an inductive probe.

In the case of a water content monitor for particulate materials, such as raw sugar crystals, the design of e* e •the cavity should be such that protrusions into the cavity are kept to a minimum. This ensures that the 10 cavity can be filled with the particulate material and emptied quickly. Thus magnetic coupling probes are preferred when the free water content of particulate materials is to be measured.

If the resonator is filled with dry sugar (or its equivalent) and the input frequency to the resonator is selected for a maximum signal from a detector connected to the output probe, the subsequent reduction in the signal from the same detector, when the resonator is filled with sugar containing some free water and an input signal at an appropriate frequency and of the same power is applied to the resonator, is an indication of the reduced Q-factor of the resona tor.

The reduction in the Q-factor is related to the water content of the sugar. Using samples of known water content, a resonator and detector monitor combination can be directly calibrated to provide an analogue indication of the water content of a sugar sample (or a sample of other material).

6 In other applications of the present invention, the water content of a material may be indicated (or determined with suitable calibration) if the resonator is such that its Q-factor can be affected by a moisture containing material in close proximity to the resonator. When the resonator is moved into the vicinity of the moisture containing material, the .change in the signal from the output probe indicates a change in the water content of the zone to which the 10 resonator is responsive.

4 Thus, according to the present invention, a method of e* obtaining an indication of the free water content of a material comprises the steps of positioning the material in, or in the vicinity of, an rf cavity resonator having an input probe and an output probe at an electric or magnetic antinode position, or at respective antinode positions, in said resonator; applying an rf signal, having a frequency 20 corresponding to the resonance frequency of the cavity resonator when it contains or is in the i vicinity of the material, to the input probe and monitoring the output signal from said output probe; and comparing the output signal obtained by step (b) with the output signal from said output probe when the cavity resonator contains or is in the vicinity of a dry sample of the material and an rf signal having a frequency corresponding to the resonance fequency of the cavity resonator containing on in the vicinity of -the dry sample is applied to said input probe.

7 A suitable cavity resonator for use at radio frequencies is the equivalent of a half-wavelength length (or a length equal to an integral number of half-wavelengths) of stripline or coaxial cable with a respective short circuit at each end of the stripline or coaxial cable, and (ii) a capacitive output probe mid-way between the short circuits.

Alternatively, the resonator may be the equivalent of a *quarter-wavelength length (or a length equal to an 10 integral number of quarter-wavelengths) of stripline or coaxial cable, with an open circuit at one end of the stripline or coaxial cable, (ii) a short circuit at the other end of the stripline or coaxial cable, and (iii) a capacitively connected output probe located at the open circuit end of the resonator or a magnetic (inductive) output probe located at the short circuit end of the resonator. The input probe, in any one of ft these constructions, may be either a capacitive probe or an inductive probe.

20 A cavity resonator having any one of these constructions, and apparatus incorporating such a cavity resonator for use in moisture determinations are also encompassed by the present invention.

Several prototype moisture detection equipments, each incorporating the apparatus of the present invention, have been used to implement the method of the present invention. Each prototype equipment includes a swept-frequency rf source. Using this rf source, it is possible to perform the method of the invention despite the de-tuning effect of various samples filling the resonator. The frequency of the peak response of the t 1 8 resonance build up and decay is captured and held, and the held value is refreshed at the sweep-rate of the rf souce.

A digital read-out system and a microprocessor have been added to one of the prototype equipments. The microprocessor allows the equipment to be calibrated from use of the equipment with two, or three, samples of known moisture content.

To further illustrate the present invention, 10 embodiments of the invention will now be described, by 0 0 way of example only, with reference to the accompanying drawings.

Brief Description of the Drawings 0t9** Figure 1 illustrates, partly as a perspective sketch and partly schematically, one arrangement (including a radio frequency resonator with capacitive input and .output probes) that may be used to monitor the water content of granular or particulate materials.

Figure 2 is another illustration of the arrangement of Figure l, with the resonator shown as viewed in the direction of arrow A.

Figure 3 is a perspective sketch (which is also partly schematic) of an alternative form of remonator, having an inductive input probe and a capacitive output probe, which is particularly suitable for use in monitoring the water content of materials which cannot 9 be~ positioned within the cavity of the resonator, or in the on-stream monitoring of ,he water content of moving granular or particulate materials.

Figure 4 is a perspective sl1,etch of a preferred form of radio frequency resonator, which has capacitive input and output probes, located at the same magnetic antinode position in the resonator.

Figure 5 is a sectional view at V-V of the resonator of Figure 4.

00 10 Detailed Description of the Illustrated Embodiments The resonator shown in Figures 1 and 2 comprises a pair of metal plates 10 joined at each end by a pair of rectangular (which may be a sqiuare) metal plates 11A and 11B. A Conductor 12, in the form of a metal strip, extends from the metal plate 11A to the metal plate 11Bl. The conductor 12 is centrally positioned between the plates 1,0 and is electrically connected to the plates IIA and IIB. A capacitive input probe 13 is mounted at the centr~e of one of the plates 1,0 andt is connected to the central conductor of a coaxial cable 17. The outer conductor C& the coaxial cable 17 is soldered or otherwise elect~rioal-ly connected to the metal plate 10 of the resonator. (in fact, in all of the embodiments illustrated in this specification, the outer conductors of the coaxial cables are electrically connected to the conducting plate on which the probe, to which the central conductor of the cable is connected, is mounted.) The coaxial cable 17 is used to connect a radio freq~uency signal to the iresonator, 10 An output probe 14 is mounted centrally on the other metal plate 10 to provide a capacitive pick-up for the radio frequency signal at the centre part of the resonator. The output probe 14 is connected via a coaxial cable 15 to a voltmeter 16.

A resonator constructed as shown in Figures 1 and 2 is equivalent to a resonant cavity formed of stripline or of a coaxial cable. The plates llA and lib act as rf short circuits and the cavity functions as a resonator when the distance d between the plates llA and lB is equal to half the wavelength (or an integral number of half-wavelengths) of the radio frequency signal input to the cavity, via probe 13.

it will be ce-ar to those skilled in this art that if 15 the radio frequency signal input to the cavity of egos Figure I is such that the cavity is a resonant cavity, then the probe 14 is located at the electric antinode of the cavity and a relatively strong detected signal will be shown by the voltmeter 16. Incidentlly, in the uses contemplated for the present invention, the power supplied to the cavity is very low, being typically less then 1 milliwatt, at a frequency of about 3 GHz.

To use the arrangement shown in Figure 1 to monitor the water content of a sample of sugar, the cavity is first filled with PTFE material. PTFE. has a dielectric constant of approximately 2, which is similar to the dielectric constant of completely dry bulk raw sugar,

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11 The frequency of the radio frequency signal connected to the cavity (this frequency should be as low as possible, commensurate with the dimensions of the cavity) is then continuously swept and the peak voltage of the signal detected at the probe 14 is noted.

Preferably a "sample and hold" swept frequency generator/detector is used for this purpose. The sensitivity of the voltmeter 16 (or other monitor of the signal detected by the probe 14) is then adjusted 10 to obtain a full scale deflection at this resonant frequency.

*0 The PTFE is then removed from the cavity, and sugar crystals are packed into it. The radio frequency signal is applied to the cavity again and its frequency is swept (while the signal strength is maintained at a constant value) until the new resonance frequency for the cavity is found. Usually the new resonant frequency is different from the resonance frequency when the cavity was loaded with PTFE unless 20 the dielectric constant of the sugar crystals is the same as that of PTFE. At the new resonant frequency of OO0 the cavity, the deflection of the voltmeter (or other monitor), which will be less than the full scale deflection observed with the PTFE filling unless the sugar is completely dry, is proportional to the Q-factor of the cavity with the sugar present. The difference between the two deflections that is, the difference between the signals detected by the probe 14 when the cavity is fild with PTFE and when the cavity is filled with sugar is due to the water content of the sugar.

12 Provided the sugar has a constant packing density, reproducible results are obtained with this arrangement. Thus the system can be calibrated, using sugar samples having a water content that is subsequently determiJned accurately using the oven drying technique outlined above, so that the f-ee water content of the sugar can be determined directly from the deflection of the voltmeter. As noted above, if the monitor of the resonance signal has a digital 10 read-out, the output from the monitor can be input to a microprocessor. When two samples of known free water content are used with the cavity resonator, the S* calibration of the equipment is determined by the microprocessor, by fitting a straight line to the calibration points. If three samples of known free water content are used, the calibration is effected by S* fitting a parabola to the calibration points.

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One disadvantage of the embodiment illustrated in Figure 1 is that it is not really suitable for use in a 20 moving stream of sugar crystals, to provide an on-stream monitor of the water content of the sugar crystals while the sugar flows through the cavity between the plates 10. When used in a moving stream of sugar crystals, the cavity tends to become clogged with the crystals. To avoid this problem, the cavity illustrated in Figure 3 was constructed.

The cavity shown in Figure 3 is the equivalent of a stripline or coaxial cable cavity of length 1 equal to one quarter the wavelength of the lowest resonant frequency of the cavity. A central conductor 22 extends between parallel conductive plates 20 which are 13 supported by an end plate 21. The conductor 22 is connected to the central conductor of coaxial cable 27 in such a manner that an inductive input probe is formed at the point where conductor 22 meets the end plate 21. This arrangement is described in more detail below, in the discussion of the embodiment of Figures 4 and 5. A capacitive output probe 24 projects through one of the plates 20 near the electric antinode region of the cavity. The signal detected by a detector 10 (diode) connected to the probe 24 is connected via a coaxial cable 25 to a monitor (voltmeter) 26.

0O "000 The probe illustrated in Figure 2 is less likely to be clogged when immersed in flowing sugar crystals or other moving granular material. However, the capacitive probe 24 is vulnerable to impact. A more rigid, yet easy to construct, cavity which uses only inductive probes has been designed by the present inventors. This preferred form of resonator is :illustrated in Figures 4 and The cavity in P3 fures 4 and 5 comprises a pair of o "parallel end plates 30 and 31 which are joined by a pair of side plates 32 and a planar, elongate conductor 33. The conductor 33 is connected to the end plates and 31 with its plane parallel to the planes of the side plates 32. The connection between the flat conductor 33 and the plate 30 is effected using a flange 34 which creates a stepped or indented region at each corner of this end of the conductor 33. A short length of wire 36 extends from a corner of the conductor 33 through an insulating block 37 in an aperture in the plate 30. The wire 36 is connected to 14 the central conductor of a coaxial cable 40, via which an input radio frequency signal can be applied to the cavity. This arrangement provides an input into the cavity formed by the plates 30, 31 and 32. Similarly, a short length of wire 38, connected to the central conductor of a coaxial cable 41, extends through an insulating block 39 in a second aperture in the plate 30 to be connected to the other corner of the conductor 33 adjacent to the flange 34. The length of wire 38 00 10 provides an inductive output probe via which a sampla of the rf signal within the cavity may be coupled, using the coaxial cable 41, to a monitor (not shown).

The (constant) width w of the central conductive strip 33 is not critical, but it should be sufficient not to affect the Q-factor of the cavity, and to minimise cross-coupling between the input probe arrangement and the output probe arrangement of the cavity. However, if the width w is too great relative to the width dimension h of the side plates 32, the 20 electromagnetic field near the open regions of the cavity will leak from the cavity.

0 0 Whether the cavity is constructed ii accordance with Figures 1 and 2, Figure 3 or Figures 4 and 5, the cavity will normally be provided with a handle to facilitate its movement to a required location and with a housing for the circuitry asscziated with the probes.

As noted at the beginning of this specification, the moisture detector can be used for purposes other than the monitoring of the water content of sugar crystals.

For example, it has been found that a monitor 15 constructed in accordance with Figures 4 and 5 can be used to determine the location of wooden studs in a stud wall lined with plaster-board, and to serve as a monitor of the moisture absorbed into the plastic material used in the nose cones of aircraft, which permit (unless the water content is too high) the transmission of airborne radar signals from the S aircraft and the reception of echoes of such signals in the aircraft.

4 In these and other industrial applications of the .a present invention in circumstances when the cavity cannot be filled with a sample to be monitored, the present inventors have found it convenient to fill the cavity with a permanent plug of PTFE ("Teflon" trade mark). The cavity is then used in the "proximity" mode, with the head of the cavity placed on the S" moisture-containing material, so that the moisture near to the cavity modifies the Q-factor of the cavity.

STho< e skilled in the art of radio frequency devices wi.l appreciate that although specific embodiments of the moisture detector have been illustrated and described in this specification, it will be possible to effect modifications to such detectors, and to use equivalent but alternative resonant cavity constructions, without departing from the present inventive concept.

Claims (15)

1. A method of obtaining an indication of the free water content of a material, said method comprising the steps of: positioning the material in, or in the S.vicinity of, an rf cavity resonator having an input probe and an output probe at an 0* electric or magnetic antinode position, or at respective antinode positions, in said resonator; S. applying an rf signal, having a frequency corresponding to the resonance frequency of the cavity resonator when it contains or is in the vicinity of the material, to the input probe and monitoring the output signal 9 from said output probe; and comparing the output signal obtained by step with the output signal from said output 4 probe when the cavity resonator contains or is in the vicinity of a dry sample of the material and an rf signal having the same strength as the applied rf signal of step(b) and having a frequency corresponding to the resonance fequency of the cavity resonator containing or in the vicinity of the dry sample is applied to said input probe.

2. A method as defined in claim 1, in which each rf signal is obtained from a swept frequency rf signal source with a "sample and hold" facility. 17

3. A method as defined in claim 1 or claim 2, in which the material is particulate material, (ii) the cavity resonator is filled with the material prior to step of claim 1, and (iii) the signal obtained in step of claim 1 is compared in step of claim 1 with a signal obtained when the cavity resonator was filled with a sample of dry particulate material or the equivalent thereof. A.

4. A method as defined in claim 3, in which tne particulate material is a moving stream of particulate material, within which the cavity resonator is positioned.

A method as defined in claim 3 or claim 4, in which said material comprises raw sugar crystals.

6. A method as defined in any preceding claim, in which each output signal is displayed on a monitor 66• and the comparison of step of claim 1 is effected by reference to a calibration of the "monitor.

7. An rf cavity resonator for use in the detection of moisture in a material, said cavity resonator comprising: an elongate electrical conductor mounted between, and parallel to, a pair of parallel, elongate, electrically conducting plates; 18 a pair of electrically conducting end plates, each end plate being connected between corresponding ends of said elongate conducting plates, each end of said electrical conductor also being connected to a respective one of said end plates; an input probe, adapted to couple an rf signal into said cavity resonator, said input probe being located at an electric or magnetic antinode position of the resonant cavity; and \e an output probe, adapted to sample the rf f signal within the cavity at the location of the output probe, said output probe being positioned at an electric or magnetic antinode position of the cavity resonator. 'O

8. An rf cavity resonator for use in the detection of *.fe moisture in a material, said cavity resonator "comprising: an electrically conducting end plate; a pair of parallel, elongate, electrically conducting plates extending in the same direction from said end plate, the elongate direction of the elongate conducting plates being at right angles to the plane of said end plate; an elongate central electrical conductor extending from said end plate between said elongate conducting plates; 19 an inductive input probe, adapted to couple an rf signal into said resonant cavity, mounted at the location of the intersection between said central conductor and said end plate; and a capacitive output probe adapted to sample the rf signal within the cavity at the position of the output probe, said output probe being mounted on one of said elongate conducting plates at a location which corresponds to an electric antinode region of said cavity resonator.

9. A cavity resonator as defined in claim 7, in which said input probe is a capacitive probe which is mounted centrally on one of said elongate )nducting plates, and said output probe is a capacitive probe which is mounted centrally on the other of said elongate conducting plates.

10. A cavity resonator as defined in claim 7, in which said elongate conductor is a plate having a width which is less than the width of said elongate conducting plates, one end of said elongate conductor is connected to its associated end plate via a flange formed integrally with the conductor but not extending the full width of the conductor, whereby said elongate conductor has a pair of corners adjacent to said flange, which are spaced from said associated end plate; 20 said input probe comprises a first length of wire which is electrically connected to one of said corners and which extends through a first insulating block in said associated end plate; and said output probe comprises a second length of wire which is electrically connected to the other of said corners and which extends through a second insulating block in said associated end plate. *o

11. An rf cavity resonator substantially as hereinbefore described with reference to Figures 1 and 2, or Figure 3, or Figures 4 and 5, of the accompanying drawings. r C

12. Apparatus for use in the detection of moisture in a material, said apparatus comprising a cavity resonator as defined in any one of claims 7 to 11; an rf signal generator connected via a first coaxial cable to the input probe of the cavity resonator; and a signal monitor connected via a second coaxial cable to the output probe of the cavity resonator,

13. Apparatus as defined in claim 12, in which said signal generator is a swept frequency generator with a "sample and hold" facility. 21

14. Apparatus as defiined in claim 12 or claim 13, in which said monitor includes a digital display and a microprocessor, the microprocessor being programmed to perform a calibration of the monitor from measurements obtained with two samples of known moisture content, by fitting the signals from the output probe to a straight line, or with three samples of known moisture content, by fitting the signals from the output probe to a parabola.

15. Apparatus as defined in claim 12, claim 13 or claim 14, in which the material is raw sugar and the cavity resonator is positioned withi a moving stream of raw sugar crystals. DATED this thirtieth day of September 1991. 0* COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION S by its Patent Attorneys DAVIES COLLISON Abstract The free water content of a material is indicated by the difference in tne Q-factor of a resonant rf cavity when it is filled with, or close to, a sample of the material in its dry state and a sample of the material being investigated. The resonant cavity comprises an elongate conductor (12, 22, 33) mounted between a pair of parallel conducting plates (10, 32), with an rf input probe (13, 36) and an rf output probe (14, 38) located at electric or magnetic antinode positions of the cavity. The input probe is preferably connected to a swept frequency rf signal generator while the output probe is connected to a monitor (16) which may be calibrated to provide a direct reading of the free water content of the material being investigated (for example, raw sugar crystals). 0