FI74816B - MAETNINGSFOERFARANDE OCH -APPARAT FOER BEROERINGSFRI MAETNING AV DAOLIGT ELEKTRICITET LEDANDE SKIV- ELLER FOLIEFORMADE MATERIALS ELEKTRISKA EGENSKAPER ELLER DESSA PAOVERKANDE ELEKTRISKA EGENSKAPER. - Google Patents

Description

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MEASUREMENT METHOD AND APPARATUS FOR MEASURING THE ELECTRICAL PROPERTIES OR PHYSICAL PROPERTIES AFFECTING ELECTRIC CONDUCTIVE PLATES OR FILM-LIKE MATERIALS

The present invention relates to a method and apparatus for measuring at radio frequencies, by means of which one or more properties of a plate-like or film-like substance can be determined simultaneously. The material can be any poorly electrically conductive material, such as plywood veneer, plastic or cellulose board, or paper. The substance passes through the sensor without touching it, allowing rapid automatic measurement. Even relatively large movements perpendicular to the plane of the material sheet or film do not significantly affect the measurement result.

Determining the properties of fast-moving plate-like and film-like materials on a production line with current measuring devices is often difficult. Sensors are often in contact with the material, such as plywood veneer moisture resistance sensors or board weight scales. With sensors that often touch the substance, for example capacitive, the perpendicular movement of the substance relative to the sensor causes a large measurement error.

15 Sensors based on transmitted radiation, such as gamma rays, are often slow and expensive, and the ionizing radiation they use poses radiation protection problems because of their hazards. Impermeable radiation, such as infrared radiation, provides information only on the very surface of the substance.

20 With the help of radio waves, it is possible to eliminate several of the above-mentioned shortcomings and sources of error. Non-contact measurement is possible, radiation is harmless and the measurement also provides information from inside the 7481 6 2 material plate. Due to the cheapness of the sensor structure itself, several radio wave sensors can also be placed in parallel, which makes fast measurement over the entire width of the 25 lines possible. By measuring with the sensor several quantities related to the propagation of radio waves in the material, it is still possible to determine several properties of the material simultaneously. However, even with radio wave sensors, the movement of the plate or film relative to the sensor often causes large measurement errors, because when measuring thin plates or films, the sensor must be brought close to the material in order to obtain a sufficient signal.

The measurement method and apparatus presented in the present invention operate on radio frequencies and thus have all the above-mentioned advantages of radio wave sensors. However, with the sensor structure a according to the invention, good measurement sensitivity is also achieved with thin materials, and the perpendicular movement of the material to be measured does not cause a large error in the result. The sensor consists of parts located on both sides of the material, which together form an electromagnetic quasi-TEM transmission line resonator with several center conductors. In this case, there are resonances of several waveforms in the structure, the quality of which and the resonance frequency are affected by the material to be measured, since its dielectric constant differs from the dielectric constant of air. By selecting the appropriate waveform, these measurement signals can be maximized while achieving a small effect of material movement on the signals. The resonant frequency and the goodness number can be measured with 45 conventional current electronics at least 50 times per second, making it possible to determine the properties of a fairly fast-moving material. By shaping the above-mentioned resonator center conductors, the measuring range of the sensor can be influenced, whereby the properties of the material can be determined locally. These properties are important, for example, when measuring the moisture content of plywood veneer, as the veneer is thin, its dielectric constant does not differ much from the dielectric constant of air when dried, and it is often strongly curved after drying. The velocity of the veneer on the production line is also quite high, up to 3 m / s, and information on its humidity is desired with a resolution of 30 x 30 cm ^ for the entire 1.5 m 55 wide line width, so the required measurement speed can be up to 50 measurements per second.

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The characteristic features of the method according to the invention are set out in claims 1 and 2 and the characteristic features of the device in claim 3.

In the following detailed description, the ra-60 structure with two center conductors is used as an example. In the description, reference is made to the following figures.

Figure 1 A measuring sensor made of two ground planes, two middle conductors and insulated on the side.

Figure 2 Bottom measuring sensor made of two ground planes, two middle conductors and insulated 65.

Fig. 3 Bottom of the central conductor strip of a butterfly-shaped measuring sensor Fig. 4 Electric fields of the even (4a) and odd (4b) waveforms of a measuring resonator comprising two central conductors as seen from the direction of propagation of the wave.

70 Figure 5 The relative change in the resonant frequency of the measuring resonator of Figures 1, 2 and 4 as a function of the location of the plywood veneer.

Sensors according to the invention which are not suitable for measuring the properties of poorly electrically conductive plate-like and film-like substances are shown in Figures 1, 2, 3 and 4. The sensor of the figures consists of two 75 ground planes A, in this example two center conductors B (strip, tube, etc.). of the pieces of insulating material C used for the suspension. The material to be measured D is in the middle of the center conductors so that the level of the material is parallel to the ground planes. The ground planes and center conductors form an electromagnetic resonator in which the electromagnetic wave 80 propagates parallel to the plane of the substance to be measured.

The resonator of Figures 1 and 2 is in resonance when the wavelength is about twice the length of the center conductors B. The dielectric plate between the center conductors B changes the wavelength and thus also the resonant frequency of the structure. The material also attenuates 85 radio waves, which can be detected by a change in the goodness of the resonator. Information on the characteristics of the resonance is obtained by connecting it to the measuring circuit with switching spikes or loops E.

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By increasing the distance between the center conductors B of the resonator according to Figs. 1, 2, 3 and 4 without changing the distance from the ground planes A 90, the sensitivity of the sensor to the change in the vertical position of the material C to be measured decreases. However, due to this law, the goodness of the resonator and the measurement sensitivity also decrease. Again, by increasing the distances of the center conductors B to the ground planes A without changing the distance between the center conductors, the measurement sensitivity is improved, but the goodness of the resonator decreases. The Herk-95's ability to move the material vertically does not change. In practice, a sufficient quality factor of about 500 is obtained when the distances of the center conductors B to the ground planes A are about 0.12 times the wavelength and the distance between the center conductors B is about 0.15 times the wavelength. In this case, the measuring sensitivity of the sensor is also good and the sensitivity to the full vertical movement of the material is low.

Quasi-TEM transmission line resonators with several center conductors, of which the resonator according to Figures 1, 2, 3 and 4 is an example, have several different waveforms with different potential combinations of center conductors. In the case of two center conductors, we can speak of a pair of Π i-105 waveforms. the center conductors are at the same potential, and the odd waveform, where the potentials are of opposite sign. Due to slightly different discontinuities, the same-order resonant frequencies of the even and odd shapes tend to differ slightly. The field patterns occurring in a plane perpendicular to the wave propagation direction of the even and odd reso-110 nance forms are also clearly different. The field pattern of the electric field of the even shape in the above plane is shown in Fig. 4a and the corresponding field pattern of the odd shape in Fig. 4b. It can be seen from the figures that when the thin plate or film to be measured is placed in the middle of the center conductors B 115, the electric field is parallel to the plane of the plate D for the substance to be measured in an even shape and perpendicular to the plate D in an odd shape. In this case, the change in the relative resonance frequency and goodness of the resonator caused by the material plate compared to the empty resonator is clearly larger in the even form than in the pa-120 form.

Also, the change in the measurement signals caused by the vertical movement of the material plate to be measured is clearly smaller in the even form than in the odd form. As an example of this, Figure 5 shows the relative resonant frequency change 125 of a resonator 7481 6 5 according to the invention compared to an empty resonator in an even shape when the plywood veneer in the resonator has been moved vertically. The curve is empirical.

When it is desired to measure e.g. the moisture of a plywood veneer, the procedure can be as follows. Both the resonant frequency change 130 of the resonator according to Figures 1, 2, 3 and 4 and the losses caused by the veneer, i.e. the so-called dielectric quality factor Q £. Then it can be shown that for different resonant forms the following holds: 1 / Qe e "(f0-fl> / f0 '(e') n (e'-1) Here fo is the resonant frequency for an empty resonator, the resonance-135 frequency when the veneer is set of Figures 4a and 4b in accordance with keskijoh-dinten midway between, ε \ veneer dielectric constant real part and ε "of the veneer di elektri syysväkion imaginary part. Constant n is zero for even the shape and one of the odd mode. therefore, the quantity of the left-hand side of the equation formed of the measurement results do not depend on the veneer or the like. 140 material thickness. the quantity also depends on the density is usually limited. For example, wood humidity in question. quantity, however, depends on, so the moisture can be determined. From the equation also found that by measuring the quantity of the left-hand side and the even-(n = 0) and odd (n = 1 ) and further calculating the ratio of these quantities 145 gives the real part of the dielectric constant of the material plate or film. The imaginary part of the dielectric constant is also determined without either result depending on the thickness of the plate to be measured.

By shaping the center conductors B of the resonator, both the resonant frequency of the re-150 and the measuring range of the sensor can be affected. For example, the butterfly-shaped strip-shaped center conductor of Fig. 3 achieves an almost twice lower resonant frequency than a flat plate center conductor of the same length. The lower resonant frequency makes it possible, for example, to increase the distance between the center conductors, thereby reducing the effect of the vertical movement of the material.

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The measuring range of the sensor extends beyond the edges of the center conductors B to a distance of about one-third of the distance between the center conductors, so butterfly-shaped center conductors have a shorter and wider measuring range than flat-width center conductors. The center wires B of the sensor 4 in Figures 1, 2, 3 and 160 are electrically open at both ends.

However, the center conductors B can be short-circuited from both ends or only one end to ground plane A without substantially changing the operation of the sensor.

Claims (3)

7481 6 7 A method for measuring the electrical properties or physical properties of poorly conductive substances (D) in the form of plates or mines, characterized in that a part of the structure is placed on each side of the substance (D) at some distance so that the parts together form an electromagnetic quasi-TEM a transmission line resonator having a plurality of center conductors (B) in which the electromagnetic quasi-TEM waves propagate in the plane of the substance to be measured (D) and in which both perpendicular (electric field at the substance to be measured (D) perpendicular to the plane of substance (D) 10) and parallel ( electric field for the measured substance (0) parallel to the plane of the substance (D)) resonances of the waveforms whose resonance is affected by the frequencies and goodness values of the measured substance (D), which are measured and the complex dielectric constant or physical properties of the measured substance (D) are solved. Measurement method according to Claim 1, characterized in that the resonant frequency and / or goodness of one of the several possible resonant modes is measured and the moisture 20 or some other physical property of the substance (D) is determined by calibration. ) the effect of perpendicular motion is considerably smaller than when using the perpendicular resonance mode in the case of single-parameter measurement, and where using the parallel resonance mode, a significantly better measurement sensitivity is achieved than when using the perpendicular resonance mode. Device for carrying out the method according to claims 1 or 2, characterized in that the resonator consists of two finite parallel ground planes (A) with two conductor bodies (B) parallel to the ground planes (A), for example a metal strip with a plate-like or hollow material to be measured. (0) are placed parallel to the ground planes (A), the measurement being affected approximately by the part of the substance to be measured (D) between the conductor pieces.