WO1999066316A1

WO1999066316A1 - Measurement of humidity by absorption of microwaves

Info

Publication number: WO1999066316A1
Application number: PCT/EP1999/003962
Authority: WO
Inventors: Josef Reitner; Alfred Weingartner
Original assignee: Mahlo Gmbh & Co. Kg
Priority date: 1998-06-18
Filing date: 1999-06-09
Publication date: 1999-12-23
Also published as: EP1088219A1

Abstract

The invention relates to a method and a device for measuring the humidity content of a flat material being measured (1) by the absorption of microwaves. The microwaves irradiate the material being measured (1) along a measuring beam chain, said chain comprising at least two measuring beams (4, 5, 6, 7), several times. Once the last measuring beam (7) has passed through, the microwaves are transferred to a detection device (11) which detects their field strength. The humidity content of the microwaves is determined from the field strength. The aim of the invention is to determine the humidity content as accurately as possible. To this end, the microwaves are linearly polarised in different directions along at least two of the measuring beams (4, 5, 6, 7). Microwave parts of other measuring beams which may have been deviated can therefore be recognised and eliminated in the respective receivers at the ends of the two measuring beams by means of their state of polarisation.

Description

Moisture measurement through microwave absorption

description

The invention relates to a method and a device for measuring the moisture content of a flat sample by microwave absorption, in particular the measurement of the residual moisture content of a continuously conveyed web, the microwaves shining through the sample several times along a measuring beam chain with at least two measuring beams and the field strength of Microwaves after

Passing through the last measuring beam the moisture content is determined. The material to be measured consists, for example, of paper, coated paper, cardboard, tissue, plastic, textile material and / or the like.

A generic method and a generic device are known from EP 0 009 187 AI. The known device has a microwave oscillator which is connected to a first horn by a microwave waveguide

Ab radiation from microwaves is connected to a measuring section. In the course of the measuring section, the microwaves emitted by the first microwave radiation strike the material to be measured. Depending on the moisture content of the sample, the thickness of the sample, the structure of the sample and the moisture distribution in the sample, the field strength of the microwaves is weakened by absorption in the sample. Furthermore, the field strength of the microwaves propagating in the direction of the measuring section is weakened by reflection, in particular on the surface of the measured material, and by diffraction or scattering. The microwaves consequently reach a microwave receiver at the end of the measuring path when the reception power is lower than the transmission power. From there, the microwaves become a second via a second microwave waveguide Microwave emitter directed, which emits the microwaves on a second measuring section, in the course of which the microwaves in turn cross the material to be measured. In a special embodiment, the previously known device has a measuring chain with a total of four successive measuring sections. At the end of the last measuring section, the microwaves are detected by a measuring detector, which transmits a signal corresponding to the remaining field strength to an evaluation device. The detector signal consists of an electrical voltage that is compared with a reference voltage. The reference voltage represents the measurement signal in the case in which no material to be measured is arranged in the device.

The multiple radiation of the measured material increases the sensitivity, so that, for example, measured material with a residual moisture content of 1 g water / m - .30 g water / m can be measured. It is known in particular in the manufacture of textiles

To control dependency on the measured values of the residual moisture content of drying machines so that a predetermined residual moisture content is achieved in the material to be measured or in the textile material.

In a special embodiment of the known device or the known method, the microwaves are modulated with a modulation frequency of 100 Hz and a stroke of more than 25%, so that fluttering movements of the measured material, which typically occur with lower frequencies than the modulation frequency in the direction of the measuring section, have no influence on the measurement result.

When carrying out the known method, microwaves can be reflected and scattered on the material to be measured from one of the measuring sections into the microwave receiver of another. Arrive at the measuring section, so that Measurement result is falsified. This is particularly the case with a special embodiment of the previously known device, in which the microwaves radiate the measured material alternately in the course of the measuring chain from one side of the measured material to the opposite side of the measured material and vice versa. Furthermore, it is also possible that microwaves from the

Microwave emitters of a measuring section can be passed directly or by deflection in another way than on the measured material into one or more microwave receivers of another measuring section.

The object of the present invention is to provide a method and a device of the type mentioned at the outset, in which the microwave signal present after passing through the last heating section or the last measuring beam largely depends only on the absorption in the material to be measured.

This task is accomplished through a process with the

Features of claim 1 or solved by a device with the features of claim 10. Further developments are the subject of the respective dependent claims.

According to a gist of the ^"present invention, the microwaves are at least the microwave chain linearly polarized along two of the measuring beams, wherein the polarization directions of the two measuring beams differ from one another, preferably an angle of about 90 ° with one another. Device side are for this purpose each to one-other opposite End points of two measuring sections, on each of which one of the measuring beams lies, a coordinated one

Microwave emitter / microwave receiver pair arranged. The microwave receiver of the first pair is via a row conductor and possibly one or several further measuring sections are coupled to the microwave transmitter of the second pair. According to the invention, the microwave emitter of the first pair (first microwave emitter) and the microwave emitter of the second pair (second

Microwave emitters) designed and aligned in such a way that linearly polarized microwaves can be transmitted on the respective measuring section, the polarization direction of the microwaves to be emitted by the first microwave emitter and that by the second microwave emitter being different.

Measuring beams are understood to mean the paths of propagation of the microwaves on which they traverse the material to be measured in the course of the measuring chain. The measuring beams can have a finite width transverse to the direction of propagation.

Due to the different polarization directions, the microwaves transmitted on the measuring sections or spreading along the measuring beams each have a property by means of which they can be selected from one another. If there are scattering, diffraction and / or reflection processes that do not depend on the absorption of the measured material,. it can be avoided that at the end of one of the at least two measuring beams, microwave signals other than those of the associated measuring section are received, forwarded and / or detected. Alternatively, a sum microwave signal which may be present at the end of a measuring section and which contains a first, linearly polarized component and a second, linearly polarized component in another polarization direction and / or unpolarized component is broken down using the polarization property, both components being evaluated. The undesired second component is used in particular to determine one caused by the respective material to be measured to determine the reflected, diffracted and / or scattered portion of the microwave radiation propagating along the measuring beam chain in order not to wrongly attribute this portion to the absorption of the microwave radiation in the material to be measured, which would lead to an overestimation of the moisture content. In this way, the measuring accuracy is increased, in particular in the case of method variants, in which, to determine the moisture content from the field strength of the microwaves after passing through the last measuring beam, a reference value is used as a comparison value which corresponds to the measuring arrangement in the absence of a measured material.

The measuring beams with the linearly polarized microwaves preferably traverse the planar measured material at an angle of 30 ° to 90 °, preferably approximately 60 °, against the surface thereof. An advantage of a non-perpendicular irradiation of the material to be measured is that reflections on the surface of the material to be measured are not reflected directly back into the microwave emitter of the respective measuring section. Standing waves can thus be avoided. Also preferred is an embodiment in which the measuring beams traverse the material to be measured at locations spaced apart from one another and in which the directions of polarization of two measuring beams that are adjacent to each other are different. In particular, two variants are proposed for this. In one variant, the points at which the measuring beams cross the material to be measured are approximately in a straight line to one another. In the other variant, the locations are approximately at the corners of a polygon, in particular a square. In the case of the square, the measurement beams preferably have different polarization directions at adjacent corners of the square. The latter case is particularly preferred if the polarization directions of the measuring beams each have two mutually adjacent corners form an angle of approximately 90 °. This has the advantage that the microwaves propagating along the two adjacent measuring beams can be selected on the basis of their polarization directions. The distance between the measuring beams at diagonally opposite corners of the square is preferably chosen to be such that at most a negligibly small extent, portions of the microwaves propagating along these measuring beams with the same polarization direction can reach the end of the other measuring section.

The determination of the moisture content from the field strength of the microwaves often depends on the temperature of the

Material to be measured, i.e. at different temperatures, a different field strength is detected at the end of the measuring beam chain, although the moisture content is, for example, the same at both temperatures. The cause of this effect is in particular one

Temperature dependence of the absorption coefficient of the moisture in the material to be measured, and a temperature dependence on undesired, parasitic effects such as, for example, absorption in the dry components of the material to be measured, diffraction, scattering and / or reflection on the material to be measured. To take this temperature dependency into account, the temperature of the measured material is preferably measured without contact and the measured temperature value is taken into account when determining the moisture content. On the device side, a thermopile, a pyroelectric sensor or a bolometer are preferably used for this.

The invention will now be explained in more detail with reference to exemplary embodiments with regard to further advantages and features. Reference is made to the attached drawing. However, the invention is not based on limited these embodiments. The individual figures of the device show:

1 shows a first embodiment of the device according to the invention in a schematic side view,

Fig. 2 is a diagram of the radiation directions and the associated directions of polarization of microwaves in that shown in Fig. 1

Contraption,

Fig. 3 is a schematic front view of a second embodiment ^'s of the inventive device, and

Fig. 4 shows a section through device parts located on the back of the device shown in Fig. 3, seen from the front.

1 shows a material web 1 which, for example, has paper, coated paper, cardboard material, plastic, textile material and / or the like sheet material, and which in the illustration shown is conveyed continuously in the direction into the image plane. The web of example, has a surface density of 3000-5000 g / m and comprises a to be measured ^"moisture content. As shown in Fig. 1 seen, the web 1 is passed through a frame-like support 10, are attached to the parts of the measuring device. The measuring device has an oscillator 9 which generates microwaves, for example with a frequency of 8-12 GHz or with a frequency of 2-4 GHz, during operation of the device The microwaves are coupled into the waveguide 8a and connected to a first horn radiator 4a the front 2 of the web 1. The first Horn radiator 4a transmits the microwaves to a transmission path along a measuring beam 4, which ends at a first horn receiver 4b at the rear 3 of the web 1, where the microwaves are coupled into and on a second waveguide 8b

Side of the web 1 are directed to a second horn 5a. In the same way, after leaving the second horn radiator 5a, the microwaves still radiate through the web 1 along a second measuring beam 5, a third measuring beam 6 and a fourth measuring beam 7. Between the respective measuring sections, they are in turn transmitted by waveguides 8c, 8d from the respective horn receiver 5b , 6b directed to the next horn emitter 6a, 7a. At the end of the last, fourth measuring beam 7

Microwaves into a fourth horn receiver 7b and from there to a detector 11, which detects the field strength of the microwaves and emits a detection signal to the computer 12, in which the temperature of the material web is determined from the detection signal and possibly other signals, such as a temperature value measured without contact 1 represents, the moisture content of the web 1 at the current conveying position on the carrier 10 is determined. Instead of the waveguides 8a, 8b, 8c, 8d, 8e, other types of microwave conductors, for example strip conductors and / or coaxial cables, can also be used.

Each time the material web 1 is traversed along the measuring beams 4, 5, 6, 7, the microwaves are subjected to partial absorption depending on the moisture content of the product web 1, so that the field strength of the microwaves is lower at the end of the measuring beam 4, 5, 6, 7 is at the beginning of the measuring beam 4, 5, 6, 7. Because of the four-fold irradiation of the web 1, on the one hand an average value of the moisture content over almost the entire width of the web 1 is determined and on the other hand the measuring sensitivity to one simply increased by radiation by approximately four times, so that even low residual moisture content values of the web 1 can be determined. Residual moisture content is understood to mean the moisture content which arises in the material to be measured due to the moisture content of its surroundings, in particular the ambient air.

In particular, in order to be able to eliminate the effects of standing waves and reflection, scattering and diffraction effects due to the web 1 fluttering transversely to the conveying direction, the microwaves coupled into the waveguide 8a are preferably at a frequency of 70 Hz with a frequency swing of 2 to 4 GHz modulated (wobbled). Interfering signals, for example from standing waves, with frequencies greater than or equal to 70 Hz, are eliminated in particular by a low-pass circuit, not shown, between the detection device 11 and the computer 12. The detection device 11, which is preferably broadband and detects in the entire transmission frequency range and has a low-pass filter connected downstream, forms an average over the transmission frequency range, which is largely free of undesired effects at certain frequencies of the

Frequency range. In particular, the disruptive effect of absorption bands of the dry sample can be eliminated. The detection device 11 has, for example, a Schottky diode for receiving the microwaves.

2, the directions of radiation of the microwaves and the respective directions of polarization on the measuring beams 4, 5, 6, 7 are shown schematically. The double arrow in the upper part of FIG. 2 shows the direction in which the web 1 is conveyed. Along the measuring beam 4, the microwaves cross the web 1 from top to bottom, ie from the Front 2, on which the first waveguide 8a lies, to the rear 3, on which the second waveguide 8b lies. The microwaves are linear along the measuring beam 4, polarized in the conveying direction of the web 1. Along the measuring beam 5, the microwaves traverse the web 1 from bottom to top, being polarized linearly, at an angle of 90 ° transversely to the conveying direction of the web 1. The crossing directions and the polarization directions along the measuring beams 6, 7 correspond to the crossing directions and the polarization directions along the measuring beams 4, 5. In this way, the polarization directions of mutually adjacent measuring beams 4, 5, 6, 7 are different, so that reflected, diffracted and / or scattered portions of neighboring microwaves

Measuring beams at the respective microwave receiver 4b, 5b, 6b, 7b of a measuring section can be selected. For this purpose, the respective horn receivers 4b, 5b, 6b, 7b are set to the respective polarization direction of the transmission path and are insensitive to microwave radiation which has a polarization direction rotated by 90 °.

In the horn radiators 4a, 5a, 6a, 7a and in the horn receivers 4b, 5b, 6b, 7b there is in each case

In the direction of the measuring section, horns widening in the manner of a funnel are provided, the cross-section of which, as cannot be seen from FIG. 1, is rectangular and is oriented in accordance with the respective polarization direction of the measuring section. The respective

The direction of polarization coincides with the orientation of the narrower side of the cross-sectional rectangle. As an alternative to horn radiators or horn receivers, patch antennas or rectangular conductor tracks are used, or other means known to the person skilled in the art for transmitting microwaves in a specific polarization direction. In a variant of the device not shown in FIG. 1, only the microwaves are linearly polarized along the middle two of the four measuring beams. Nevertheless, any undesirably deflected microwaves from adjacent measuring sections can be effectively selected.

Fig. 3 and Fig. 4 show another measuring arrangement with horns 14a, 15a, 16a, 17a and with horn receivers 14b, 15b, 16b, 17b, which are arranged at end points of four measuring sections, each in pairs opposite to each other on a conveyor section of the web 1 are. Fig. 3 shows the arrangement on the front, Fig. 4 on the back of the web 1, each seen from the front.

The guidance of the microwaves is as follows: From the oscillator 9, the microwaves are coupled into the first horn antenna 14a via a microwave conductor, in particular a coaxial cable. As can be seen from the illustration, the first horn radiator 14a is equipped with a horn 18 which has a rectangular cross section which widens in the manner of a funnel in the direction of the measuring section, the longer side of the rectangle in FIG

Direction of polarization (straight arrow to the left) points. The horn radiator 14a emits microwaves with this polarization direction onto the measuring section. At the end of the measuring section, the microwaves are received by a horn receiver 14b with a horn 18 oriented in the same direction. Both on the front and the horns are on the back of the web ^'18 arranged and oriented such that the total of four measuring paths pass through the web 1 at locations that define approximately the corners of a square. Of the

Horn receivers 14b now guide the microwaves via a microwave guide to the horn radiator 15a, which lies at one end of the measuring section that of the first Measuring section is diagonally opposite. The horn 15a and the associated horn receiver 15b have the same orientation as the horn 14a and the horn receiver 14b, ie the microwaves on the two associated measuring sections are linearly polarized in the same direction.

It has been observed that when a flat sample is irradiated once, the microwaves are weakened to different extents from the front to the back of the sample or vice versa, with the same moisture content of the flat sample. A possible reason for this is a different structure on the front surface and on the back surface

Surface of the material to be measured. Another possible reason lies in a layer-like structure of the measured material which is asymmetrical with respect to the direction of propagation of the microwaves. By the measuring arrangement shown in FIGS. 3 and 4, in which the microwaves are the same

Direction of polarization along measuring beams immediately following one another in the measuring beam chain, first irradiating the measured material in one direction and then immediately irradiating the measured material in the opposite direction, direction-dependent effects such as those described above can be compensated or a suitable mean value can be formed. In particular, if the field strength of the microwaves is only weakened by a small percentage when the sample is irradiated, it is irrelevant in which direction the sample is first irradiated.

In the further course of the measuring chain, the microwaves on the front of the web 1 are directed from the horn receiver 15b via a microwave guide to the horn radiator 16a, which emits the microwaves on the third measuring path to the horn receiver 16b. The microwaves are then passed to the fourth and last measuring section, at the end points of which the horn radiator 17a and the horn receiver 17b are arranged, via a further microwave guide on the back of the web 1. With rotated by 90 °

The direction of polarization with respect to the horn radiators 14a, 15a and horn receivers 14b, 15b of the first and second measuring sections, the horn radiators 16a, 17a and the horn receivers 16b, 17b of the third and fourth measuring sections are again the same

Polarization direction set. The third and fourth measuring sections cross the web 1 again at diagonally opposite corner points of the imaginary square. At the end of the fourth measuring section, the microwaves are emitted by the horn receiver 17b

Microwave guide passed to the detector 11. The detection signal is evaluated in particular in the same way as in the device shown in FIG. 1. Compared to the device shown in Fig. 1, the device of Figs. 3 and 4 has the advantage that the measuring arrangement has a smaller width, i.e. the same distance between the next adjacent measuring sections. Material webs with a smaller width can be measured. Furthermore, the spatial averaging of the moisture content over areas of smaller width is possible, whereby the same distances between the next adjacent measuring sections can be maintained.

In the exemplary embodiments shown, the

Microwaves radiated through the measuring material via a chain of measuring sections with a total of four successive measuring sections. However, the invention is not limited to a measuring section chain with four chain links (measuring sections). Rather, the measuring section chain can have any number greater than or equal to two measuring sections. The number depends in particular on the desired sensitivity. Reference list

Material web 8b second waveguide front 8c third waveguide rear 8d fourth waveguide first measuring beam 9 oscillator a first horn 10 carrier b first horn receiver 11 detector second measuring beam 12 computer a second horn radiator 14a horn radiatorb second horn receiver 14b horn receiver third measuring beam 15a horn radiatora third horn radiator 15b horn receiver b third horn receiver 16a Horn emitter fourth measuring beam 16b horn receivera fourth horn emitter 17a horn emitterb fourth horn receiver 17b horn receivera first waveguide 18 horns

Claims

claims

1. A method for measuring the moisture content of a flat measurement object (1) by microwave absorption, in particular for measuring the residual moisture content of a continuously conveyed web, the microwaves measuring the object (1) along a measurement beam chain with at least two measurement beams (4, 5, 6, 7) Pass through several times and the moisture content is determined from the field strength of the microwaves after passing through the last measuring beam (7), characterized in that the microwaves are linearly polarized at least along two of the measuring beams (4, 5, 6, 7), the Distinguish the polarization directions of the two measuring beams (4, 5) from each other.

2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that the polarization directions of the two measuring beams (4, 5) enclose an angle of about 90 ° with each other.

3. The method according to claim 1 or 2, d a d u r c h g e k e n n z e i c h n e t that the measuring beams (4, 5, 6, 7) cross the flat material to be measured (1) at an angle of 30 ° to 90 °, preferably about 60 °, against its surface.

4. The method according to any one of claims 1 to 3, characterized in that the measuring beams (4, 5, 6, 7) traverse the material to be measured (1) at spatially spaced locations and that the directions of polarization of two adjacent measuring beams (4, 5) are different.

5. The method of claim 4, d a d u r c h g e k e n n z e i c h n e t that the measuring beams traverse the material to be measured (1) at points which are approximately in a straight line to each other.

6. The method of claim 4, d a d u r c h g e k e n n z e i c h n e t that the spaced apart locations are approximately at the corners of a polygon.

7. The method according to claim 6, wherein the polygon is a square, so that the measuring beams have different polarization directions at adjacent corners of the square.

8. The method according to any one of claims 1 to 7, so that the microwaves alternately measure the material to be measured (1) along the measuring beam chain from one side of the

Radiate the material to be measured (1) to the opposite side of the material to be measured (1) and vice versa.

9. The method according to any one of claims 1 to 8, wherein the determination of the moisture content from the

Field strength of the microwaves is dependent on the temperature of the measured material (1), characterized in that the temperature of the measured material (1) is measured without contact and that the measured temperature value is taken into account when determining the moisture content.

0. Device for measuring the moisture content of a flat measured material by microwave absorption, in particular for carrying out the method according to one of claims 1 to 9, with - a first microwave radiator (4a) and a second microwave radiator (5a), a first microwave receiver (4b) and a second microwave receiver (5b), a detection device (11) for detecting the field strength of microwaves and an evaluation device (12) connected to the detection device (11) for determining the moisture content from the detected field strength, the first microwave radiator (4a) and the first Microwave receivers (4b) and the second microwave receiver (5b) are each arranged at mutually opposite end points of a microwave measuring section and are matched to one another, the first microwave receiver (4b) being connected via a microwave conductor (8b) and optionally via one or more further measuring sections the two th microwave oven (5a) is coupled and the second

Microwave receiver (5b), if necessary, is coupled to the detection device (11) via one or more further measuring sections, so that the first (4a) and the second (5a)

Microwave emitters are designed and aligned in such a way that linearly polarized microwaves can be transmitted on the respective measuring section, the polarization directions being different from the first

Differentiate between the microwave emitter (4a) and the microwaves to be emitted by the second microwave emitter (5a).

11. The device according to claim 10, characterized in that the microwave radiator (4a, 5a) and the microwave receiver (4b, 5b) each have a funnel-like, widening in the direction of the measuring section horn (18), the cross section of which is rectangular and that according to the direction of polarization the respective measuring section is aligned.

12. The apparatus of claim 10 or 11 for

Carrying out a method with the features of claim 9, so that a thermopile, a pyroelectric sensor or a bolometer is provided for measuring the temperature of the measured material (1).