AU759006B2

AU759006B2 - Microwave reference improvements

Info

Publication number: AU759006B2
Application number: AU17421/99A
Authority: AU
Inventors: Richard Kingswood Kelly
Original assignee: Vomax Pty Ltd
Current assignee: Vomax Pty Ltd
Priority date: 1998-02-20
Filing date: 1999-02-22
Publication date: 2003-04-03
Anticipated expiration: 2019-02-22
Also published as: AU1742199A

Description

I I

U

48641 HKS/BM P/00/009 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

0

A

*0 0**0 0@ 0000 0 *00* 0* *000 ~0 00 A 00 0 0* *0 0000 0 0000 0* 0 0 C

A.

00 0 00 00 COMPLETE SPECIFICATION FOR AN INVENTION Invention Title: MICROWAVE REFERENCE IMPROVEMENTS Name of Applicant: VOMAX PTY LTD Address for Service: COLLISON Co. 117 King William Street, Adelaide, S.A. 5000 The invention is described in the following statement 2 This invention relates to apparatus and methods relating to assessment of characteristic of materials which can be discerned by the use of microwave energy.

The use of microwave based instruments for the measurement of material properties is known.

An ideal way to ensure that a microwave based instrument is operating within acceptable limits for accurate repeatability is to use as a reference a typical sample of the normal process material which is to be measured.

In a typical instance which is not to be considered as a limiting example the 1 0 measurement can be of a bale of cotton where the moisture content of the cotton in the bale is to be assessed by measuring a degree of change to the S. microwave energy directed to pass through the bale.

.For most accurate results it is considered to be very beneficial to use a **:reference or standard bale by which any instrument to be used for assessment 1 5 can be calibrated. For such a standard or reference bale to be used as a long term reference the assay for the sample must be known and the sample sealed in a way which will not allow the properties of the sample to change with time, including spatial distribution of properties. Any sealing process must also not have any significant effect on the measurement.

Such an ideal sample in practical circumstances is rarely achievable.

Calibration checks often require that sample readings are typically taken from the process at the same time as the instrument readings are taken and regression analysis carried out between the laboratory assays of the process sample readings and the instrument readings to determine an extent of accuracy of the readings.

In such a process it is difficult to attribute sources of error, and any drift in the instrument is usually combined with other sources of error such as sampling error and laboratory analysis error.

One technique which has been used to isolate sources of error in a sampling program is the "Grubbs Estimator" technique.

Typically, a microwave based instrument is being used to measure the moisture content of the process material such as cotton, and a typical sample of the process material will change in moisture content with time and its surrounding environmental conditions such as temperature and humidity.

Sealing containers of a sample of the typical process material is no guarantee of homogeneity of moisture throughout the material and as a sample reading is reading only a part of the bale then there is a chance of substantial inaccuracy.

It is an object of this invention to propose a better answer than has been hitherto available for providing an accurate, convenient and economic reference or calibration for these purposes.

The invention results from an understanding of microwaves and ways in which 1 5 these can be better used for the purpose.

In one form of this invention it can be said to reside in a reference apparatus for providing a bale equivalent standard reference for reference or calibration purposes for a microwave based bale moisture measuring system, the reference apparatus having a guide or guides in the form of a horn or horns to 2 0 act as a reverse antenna or antennae, to direct a microwave signal, when appropriately aligned therewith, transmitted from a moisture measuring system into and subsequently away from a microwave waveguide or guides which is or are appropriately dimensioned for a selected microwave frequency to be used by the measuring system, with the waveguide or guides arranged to effect direction of microwave energy through means adapted to effect change to a selected extent of attenuation of the signal and a selected extent of delay of the signal.

This is useful because a waveguide for useful microwave frequencies is implicitly a size in cross section that is somewhat smaller than an area of initial 3 0 transmitted signal which is used for ongoing measurements of moisture content of products such as bales of cotton.

Accordingly, a concentration of microwave energy will allow for a reduction of all up weight of the reference apparatus. Further, by effecting this concentration reduces or removes any problem of lack of homogeneity of moisture distribution because the signal will implicitly pass through all of the sample or any means providing an equivalent electrical attentuation and delay. Furthermore, it is more economic because the apparatus can be made to have significantly reduced weight and size.

Such an apparatus may be used as a reference to check the measurements made by a microwave based transmission or reflection instrument through 1 0 material which exhibits similar microwave properties to the process material to be measured, but may have the advantage of having stable microwave characteristics and made easier to handle through lower weight, mechanical stability or difficulties of containment of a suitable sample of the normal process material.

One possible solution is to make a block out of stable material which has the same electrical properties as the typical sample of the process material. In many instances such a block would be too large, heavy or unwieldy to be manually handled. This invention offers a solution to the problems of achieving stable properties with ease of handling for testing microwave based instruments.

Although this invention is useful in open path systems such as material on a conveyor belt or other similar conveying system, it could also be used with a microwave based instrument where the material under test is in an enclosed microwave transmission or reflection configuration.

Typically the reference apparatus which is the subject of this invention would be used with microwave based instruments for measuring process parameters. Microwave based instruments for measuring process parameters have been used by industry for many years, and in some cases many decades.

3 0 Prior art includes microwave only transmission and reflection type instruments which use either or both microwave velocity measurement (often referred to as phase measurement) and microwave attenuation. The combination of velocity (or delay) and attenuation measurements are often used to obtain a density corrected measurement of a process property (usually moisture content).

Alternatively, other techniques to measure the bulk density (or mass) in the microwave beam such as gamma ray transmission have been used to obtain a density compensated moisture reading in combination with either microwave velocity or microwave attenuation measurements.

An object of the invention can be said to be to provide a better reference system for users of microwave measuring gauges on process materials with a stable reference with which to check the calibration of their gauges.

In one form of this invention this can be said to reside in a reference apparatus 1 0 for calibration of a microwave transmission, receiving and measuring instrument the reference apparatus including a microwave guide with a microwave receiving means to guide microwave transmission from the instrument through a microwave waveguide, the microwave waveguide being adapted to receive and hold a reference material for effecting selected change 1 5 in transmission of any microwave therethrough, and means to transmit resulting microwaves either by way of continued passage through a microwave guide or by reason of reflection back through a return path to receiving means for the receiving means of the instrument.

In a further form of the invention this can be said to reside in a method of 2 0 calibrating a microwave transmission, receiving and measuring instrument comprising the steps of aligning a reference apparatus with the microwave transmission, receiving and measuring instrument in a calibrating position where the reference apparatus includes a microwave guide with a microwave receiving means to guide microwave transmission from the instrument through a microwave waveguide, the microwave waveguide holding therein a reference material for effecting selected effect any microwave therethrough, and means to transmit resulting attenuated microwaves either by way of continued passage through a microwave guide or by reason of reflection back through a return path to receiving means of the reference to the receiving 3 0 means of the instrument.

In a preferred but not essential feature the reference apparatus includes an outer container which houses within it active elements as above.

6 In preference in one embodiment an active part of a reference apparatus consists of two microwave antenna arranged in such a way that the microwave signals are guided into a microwave waveguide section positioned between the two antennae so that one receives the signal from the transmitter antennae and the other carries the signal, once through the waveguide to a location to be received by the receiver of the instrument.

Alternatively, in a reflective configuration only one antenna is used and the waveguide section is terminated appropriately. The antenna or microwave waveguide or both the antenna and the microwave waveguide are filled, or 1 0 partially filled with materials (or not filled at all because air is providing the appropriate effect) which will give the standard the same or similar microwave electrical characteristics as a typical sample of the normal process material.

Alternatively, the standard may be made to give microwave characteristics outside of the typical range of the process material if a wider dynamic range of 15 reference is required.

For a better understanding of this invention this will now be described with reference to a preferred embodiment which shall be described with the assistance of drawings in which FIG 1 is a schematic view of an arrangement of a typical microwave 2 0 measuring instrument with a bale in a to be measured position; FIG 2 is the same view as in FIG 1 except that there is now located a reference apparatus with a dotted outline of some parts of the reference ,apparatus in accord with this first embodiment; FIG 3 is an exploded view of an apparatus in accord with the first embodiment FIG 4 is a perspective view of an active part of the apparatus with a part cutaway to illustrate the location of reference sample material and its possible location in accord with the first embodiment; FIG 5 is a side elevation schematic of a second embodiment using a 3 0 reflected arrangement.

FIG 6 is a side elevation schematic of a third embodiment using electrical components providing an equivalent attentuation and delay.

In the first embodiment microwaves being a chosen single frequency in the "ISM" radio frequency bands are transmitted from the microwave based instrument 1 through a transmission horn 2 across air gap 3 and into a microwave horn antenna 4.

This antenna 4 has a shape and size that firstly is appropriate for the microwave frequency being used and then has a receiving end 5 to suit the transmission outlet location shape and position from the instrument 1. The microwave signal is guided by the microwave receiving means namely the horn antenna 4 with a larger end outermost directing microwave energy into a smaller innermost end to which is attached a microwave waveguide 6. The size and shape in cross section of the waveguide 6 are governed by the frequency of the signal. The waveguide 6 is comprised of conventional 15 reflective material which has defined internal dimensions leaving a hollow passageway 7 therethrough. In this embodiment the passageway 7 is filled with a foamed plastics material 20 which is inserted into the passageway from one end. This material is chosen from many available to give similar S°characteristics eg attenuation and or delay.

The frequency of signal chosen for the embodiment is 2.425 gigahertz and the reference material has been chosen to provide 22dB attenuation and 3.7 nanoseconds of delay. These are chosen to be close to those that will be experienced with cotton bales in ginning plants which are producing high :density cotton bales with a typical moisture content of from 4 to The microwaves are guided out from the microwave waveguide passageway 7 by a horn antenna 8 which has a smaller end innermost and attached to the wave guide and a larger end outermost out from which the microwave energy is directed to then cross air gap 9 to the receiving antenna 10 of the microwave based instrument 1.

3 0 In the first embodiment of the invention, a container 11 is fabricated in fibreglass and is used to support the active parts of the reference apparatus in a way that is both convenient to handle and gives the correct vertical placement of the guidance system in relation to the microwave based 8 instrument used for measuring process parameters when placed on the process material conveying system.

Using this arrangement leaves only the requirement that the positioning in the two horizontal directions to position and substantially align the reference apparatus with the respective antennas correctly is required. Such alignment accuracy is established by trial and error.

In this embodiment microwave absorbing material 12 is located around the microwave horn antenna 4 and 8 at each end to minimise stray microwave radiation, and alternative microwave paths from interfering with the operation 1 0 of the reference apparatus.

In a second embodiment a reference apparatus has a single receiving and transmitting horn antenna 14 with a larger end outermost and to which a microwave waveguide 15 is attached. The waveguide 15 however is terminated with a microwave reflector 16. The instrument is accordingly of a ,0 15 reflective type.

Again however the waveguide 15 is hollow and has selected material (or none) in the waveguide to effect selected attenuation and delay.

.e In practice both embodiments provide significant improvements to that which has hitherto been available for reference purposes.

In a third embodiment shown in Figure 6 there are two horn antennas 22 which are arranged with their wider ends outermost and with their inner ends 23 connected to in each case a respective hollow wave guide as shown at 24 in the one case and 25 in the other.

Th horns 22 with their respective waveguides 24 or 25 are held together by a slidable clamping arrangement 26 which include a first tubular element 27 which is slidable within a section tubular element 28 which are held in a relatively extended position by bolt 29. This allows for relative adjustment of the width overall of the referenced apparatus which assists for alignment purposes.

The microwave signal however is detected by an antenna 30 in one case and 31 in the other which passes into the waveguide 24 in the one case and waveguide 25 in the other. The signal is then fed through a coaxial line 32 to electrical components which will provide, through a delay component 33, a selected degree of delay and through an attentuation component a selected degree of attentuation so that, together with the inherent delay and attentuation of the horn antenna and the wave guide and couplings there will be an equivalent effect for a selected degree of moisture in a cotton bale.

The advantages previously achieved of smaller size and effective homogeneity still apply and this third embodiment then offers a further advantage that the components may retain their values for even longer than a treated sample.

Claims

1. A reference apparatus for providing a bale equivalent microwave delay and attentuation for reference or calibration purposes for a microwave based bale moisture measuring system, the reference apparatus having a first waveguide or waveguides and an antenna or antennae, to direct a microwave signal, when appropriately aligned therewith, transmitted from a moisture measuring system into and subsequently away from a second microwave waveguide or waveguides which is or are appropriately dimensioned for a selected microwave frequency to be used by the measuring system, with the second waveguide or waveguides arranged to effect direction of microwave energy through means adapted to effect change by reason of a selected extent of attenuation of the signal and a selected extent of delay of the signal. o

2. A reference apparatus as in claim 1 further characterised in that the o•••o means adapted to effect a selected extent of attenuation of the signal and a selected extent of delay include some of a same material as that within bales the moisture content of which is to be measured.

3. A reference apparatus as in claim 2 further characterised in that the material is cotton.

4. A reference apparatus as in claim 1 further characterised in that the *25 means adapted to effect a selected extent of attenuation of the signal and a selected extent of delay of the signal include electrical components to effect such selected extent of attenuation and delay from a microwave signal derived from within the second waveguide.

A reference apparatus as in claim 4 further characterised in that the electrical components include a microwave attenuator and a microwave signal delay component.

6. A reference apparatus as in any one of the preceding claims further characterised in that there is an outer container of similar external dimensions and shape to a bale of a size and type to be assessed by the measuring apparatus which container houses all of the active parts of the reference apparatus as characterised in any one or more of the preceding claims. 11

7. A reference apparatus for providing a bale equivalent standard reference for reference or calibration purposes for a microwave based bale moisture measuring system, the reference apparatus having two microwave antenna arranged in such a way that any microwave signal is guided into a microwave waveguide which is appropriately dimensioned for a selected microwave frequency to be used by the measuring system, with the waveguide arranged to effect direction of microwave energy through means adapted to effect a selected extent of attenuation of the signal and a selected extent of delay of the signal, the microwave guide being positioned between the two antennae whereby one antenna is adapted to receive a signal transmitted from a transmitter antenna when substantially coincident therewith and the other antenna is adapted to transmit any signal once through the waveguide to a receiving antenna of the measuring apparatus while substantially coincident therewith.

8. A reference apparatus for providing a bale equivalent standard reference for reference or calibration purposes for a microwave based bale moisture measuring 20 system, the reference apparatus having one microwave antenna arranged in such a way that any microwave signal is guided into a microwave waveguide which is appropriately dimensioned for a selected microwave frequency to be used by the i~ :measuring system, with the waveguide arranged to effect direction of microwave Senergy through means adapted to effect a selected extent of attenuation of the signal and a selected extent of delay of the signal, the microwave guide being positioned and otherwise adapted to effect a reflection of the signal back through the same antenna to be received by the receiver of the instrument.

9. A reference apparatus as in any one of the preceding claims wherein microwaves chosen are a single frequency in the "ISM" radio frequency bands.

A reference apparatus as in any one of the preceding claims further characterised in that there is a passage way extending through respective waveguides and at least one of these passageways is filled with a foamed plastics material.

11. A reference apparatus as in any one of the preceding claims further characterised in that the microwave frequency to which the waveguide is adapted to be effective as a guide for is 2.425 gigahertz and the means adapted to effect change of the signal will effect 22dB of attenuation and 3.7 nanoseconds of delay. 1 0

12. A reference apparatus as in any one of the preceding claims further characterised in that there is included a microwave absorbing material surrounding the entrance to the microwave receiving means and microwave absorbing material surrounding an exit from the microwave transmitting means or microwave absorbing material surrounding both the entrance to the 0oo 15 microwave receiving means and the exit from the microwave transmitting means. :00000

13. A reference apparatus as in any one of the preceding claims further characterised in that there is a first horn attached to a first waveguide and a 00.4. second horn attached to a second waveguide, and there are means to enable 20 the respective waveguides with their attached horns to be moveable apart, and means including electrical components providing transmission of :electrical microwave signals with a selected extent of attentuation and delay 0 from the one wave guide to the other wave guide.

14. A method of calibrating a microwave transmission, receiving and 0 25 measuring system including the steps of aligning a reference apparatus as characterised in any one of the preceding claims so that a calibration signal is directed into the reference apparatus and returned to the measuring apparatus, the signal being changed by having a selected extent of attentuation of the signal and a selected extent of delay of the signal imposed within the reference apparatus so that this returned signal can be used by the measuring system to effect a calibration of the system.

A method of calibrating a microwave transmission, receiving and measuring system as in the immediately preceding claim which is for 13 measuring an extent of moisture in a cotton bale.

16. A reference apparatus substantially as described in the specification with reference to and as illustrated by any one or more of the accompanying drawings.

17. A method of effecting a reference substantially as described in the specification with reference to and as illustrated by any one or more of the accompanying drawings. Dated this 22nd day of February 1999 VOMAX PTY LTD 1 0 By their Patent Attorneys, COLLISON CO. ee *t