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EP2009427A2 - Device and method for mass and / or moisture measurement of dielectric objects by determining the quality and frequency of at least two modes of a microwave resonator - Google Patents

Device and method for mass and / or moisture measurement of dielectric objects by determining the quality and frequency of at least two modes of a microwave resonator Download PDF

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Publication number
EP2009427A2
EP2009427A2EP08009786AEP08009786AEP2009427A2EP 2009427 A2EP2009427 A2EP 2009427A2EP 08009786 AEP08009786 AEP 08009786AEP 08009786 AEP08009786 AEP 08009786AEP 2009427 A2EP2009427 A2EP 2009427A2
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EP
European Patent Office
Prior art keywords
resonator
frequency
modes
resonance
measured
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EP08009786A
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English (en)
French (fr)
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EP2009427A3 (de
Inventor
Rainer Herrmann
Udo Schlemm
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TEWS Elektronik GmbH and Co KG
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TEWS Elektronik GmbH and Co KG
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Publication of EP2009427A3publicationCriticalpatent / EP2009427A3 / de
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Classifications

    • G — PHYSICS
    • G01 - MEASURING; TESTING
    • G01N — INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N22 / 00 — Investigating or analyzing materials by the use of microwaves or radio waves, i.e. electromagnetic waves with a wavelength of one millimeter or more
    • G01N22 / 04 — Investigating moisture content

Abstract

Description

  • The present invention relates to a device and a method for measuring the mass and / or moisture of dielectric objects.
  • With regard to the measurement of the mass and / or the moisture of dielectric objects, the invention is based on microwave technology, known per se, in which the object to be measured is introduced into a resonator. Due to its dielectric properties, the object changes an electromagnetic resonance impressed on the resonator. The mass and humidity of the dielectric object are then determined from the change in the resonance curve and the shift in the resonance frequency.
  • A method for measuring the mass and / or moisture of the contents of capsules is known from US Pat. A measuring device which has at least two resonators is provided for the measurement. The shift in the resonance frequency (A) caused by the capsule and the broadening of the resonance curve (B) is determined and evaluated in both resonators. The first resonator has a measuring field that is homogeneous over the capsule dimensions for determining the total mass and / or humidity of the capsule. In the second resonator, in which the capsule is guided through a format-dependent sample guide, the capsule contents are not distributed homogeneously in the capsule due to gravity, but are located in a part of the capsule with which a narrow measuring field for determining a location-dependent profile of the mass and / or moisture is passed through. If the capsule format is changed, the measuring arrangement must be converted with a new format-dependent sample guide.
  • A method and a device for determining the mass of portioned units of active ingredient are known. In the process, capsules, tablets or coated tablets are passed through a microwave resonator, which determines a shift in the resonance frequency and a broadening of the resonance curve. The measured values ​​are used to determine the mass with compensation for the influence of moisture, the mass being assumed to be directly proportional to the shift in the resonance frequency and directly proportional to the broadening of the resonance curve. It has been found, however, that the results always remain subject to a certain degree of inaccuracy in some applications.
  • A measuring device having a spherical resonator is known from US Pat. Two identical resonance modes are fed into the resonator, which have essentially the same resonance frequency but different field orientations relative to one another. The measurement setup should serve to determine the mass of elongated filaments. To evaluate the results, a difference in the resonance frequency of the two modes is used. The difference in the resonance frequencies is sensitive to both the mass and the humidity, so that a mass-independent humidity measurement is not possible.
  • A method for classifying objects which are introduced into a resonator is known from US Pat. The objects to be classified are penetrated by as strong an electric field as possible in the center of the resonator. In order to be able to measure the sample independently of its position in the center of the resonator, the microwave radiation from the different directions is superimposed so that it differs in the center of the resonator to form a field of maximum field strength. As the sample moves through the superposition of three orthogonal fields with approximately equal resonances, the shape and orientation of the sample is averaged and the result is independent of the position of the sample. The information is given by the rotation of the electric field vector and does not represent an independent analysis in different spatial directions. The disadvantage of this classification method is that only spatial mean values ​​can be measured and the measurement resolution is therefore limited. Another disadvantage is that the method only works if the specimen is placed exactly in the center of the resonator. Additional sensors are required for this.
  • An arrangement for determining the distribution of the complex permittivity of an examination object is known from FIG. Monofrequency microwaves are fed into the resonator and the amplitude and phase of the transmitted and reflected signals are evaluated. The resonance of the resonator is not evaluated. During the measurement, the material to be measured rests in the resonator, which only has the function of shielding. The spatial distribution of the permittivity is examined in that microwaves are coupled in and out at different positions.
  • The invention is based on the object of providing a device and a method for measuring the mass and / or moisture of dielectric objects, which allows a quick and precise measurement of the dielectric objects with simple means.
  • According to the invention the object is achieved by a device having the features according to claim 1. The object is also achieved by a method having the features according to claim 15. Advantageous refinements of the method and the device are the subject of the subclaims.
  • The device according to the invention is used to measure the mass and / or moisture of dielectric objects. The device uses microwave resonance technology and has an evaluation unit, at least one high-frequency generator, at least one high-frequency detector and a high-frequency resonator. The electromagnetic fields generating the resonance are generated via the at least one high-frequency generator and fed into the high-frequency resonator. The electrical and / or magnetic properties of resonances in the high-frequency resonator can be measured via the at least one high-frequency detector. According to the invention, the at least one high-frequency generator generates at least two mutually independent modes with different resonance frequencies in the high-frequency resonator. In this context, independent modes means that when the first mode is excited, little or no excitation is generated for the other modes and the electric fields of the modes enclose an angle with one another that differs from 0 ° and 180 °. The directions of the electric fields are not congruent, but oriented in different, mutually independent spatial directions. Preferably three fields are perpendicular to one another. Furthermore, the at least one high-frequency detector can measure the frequencies that occur for each mode in the resonator. The high-frequency detector is able to measure the frequencies occurring separately in each mode, the measurement of the frequencies making it possible to determine a resonance curve and the resonance frequency. The evaluation unit determines a shift in the resonance frequency and a change in the resonance curve for the measured frequencies of each mode. From the specific values ​​for the shift in the resonance frequency and for the change in the resonance curve, the values ​​for the mass and / or humidity of the dielectric object are determined independently of one another in the resonator; the specific values ​​are independent of the position, the type of movement and the specific Shape of the object. A special feature of the device according to the invention is that a plurality of mutually independent modes are generated in a resonator and evaluated independently of one another. The invention is based on the knowledge that the systematic fluctuations in the measured values ​​resulting from a known structure are due to the different position of the measurement object. If, for example, a dielectric object is considered that has different dimensions with respect to two axes and thus different mass and moisture distributions, the measurement result depends on the position of the object to be measured relative to the electric field. By using two or more independent modes, the measuring process can be carried out in mutually independent directions of the object to be measured and therefore compensate for different spatial orientations of the sample during the movements and different sample shapes along the measured directions. A measurement with two independent modes also brings a significant improvement over a single-mode measurement.
  • Furthermore, the type of movement of the test specimen through the measuring resonator only plays a subordinate role, so that a guide that is dependent on the format of the test specimen is not absolutely necessary. This not only eliminates the need for time-consuming conversion when changing the format of the production process on the device, but it is also possible to increase the throughput of test specimens per unit of time without mechanical restriction. This means that even at production rates of up to 10 ^ 6 test specimens per hour, one hundred percent control of the mass and moisture of each individual test specimen is possible.
  • At least two high-frequency generators are preferably provided, the resonance frequencies of which are each different from one another. A separate high-frequency generator is preferably provided for each resonance mode with its resonance frequency.
  • In a preferred embodiment, the evaluation unit determines, as a change in the resonance curve, a broadening of the resonance curve in the region of the resonance frequency. Alternatively, it is possible for the evaluation unit to determine the change in the resonance amplitude as a change in the resonance curve. It is also possible to use a combination of the change in the resonance amplitude and the broadening of the resonance curve for the evaluation. For the evaluation, the change in the resonance curve is combined with a shift in the resonance frequency caused by the dielectric object that has entered.
  • In one possible embodiment, the at least one high-frequency detector can determine the frequencies occurring in the resonator several times during a measurement cycle. This means that the change in the electromagnetic field is not only measured once during a measurement cycle, but that a large number of measurement processes are carried out during a measurement cycle.
  • There are different approaches with regard to the evaluation of the measurement results for the different modes. In a preferred approach, the evaluation unit evaluates the change in the resonance curve and the shift in the resonance frequency for each mode at the same time. This means that the measurement results of all three modes are evaluated at a common point in time. In an alternative approach, the evaluation unit evaluates the change in the resonance curve and the shift in the resonance frequency for each mode independently. Both approaches to evaluation make it possible to obtain position-independent measurement results for the dielectric object.
  • In a preferred embodiment, each mode has a resonance frequency that is different from the resonance frequencies of the other modes. The resonance frequencies of the modes preferably have a minimum frequency spacing in pairs, which is preferably at least 100 MHz. The distance between the resonance frequencies of the individual modes can ensure that the signals for each resonance frequency can be evaluated undisturbed by the signals of the other resonance frequencies. In this way, an independent analysis of the dielectric object is possible in each spatial direction, which is predetermined by one of the modes, and averaging can be avoided.
  • In a preferred embodiment, precisely one high-frequency generator and precisely one high-frequency detector are provided for each mode. In an alternative embodiment, at least one of the high-frequency generators is provided for generating several modes. It is also possible to provide a high-frequency detector which can measure the frequencies occurring for several modes. In a preferred embodiment, the high-frequency detector and / or the high-frequency generator can be switched between the modes, so that a large number of measured values ​​can be recorded during one measuring cycle.
  • In a preferred embodiment, a phase shifter is provided which is connected to a high-frequency generator for generating a rotating electrical field in one plane. The rotation of the electric field is generated by the timing of the phase shifter.
  • In the device according to the invention, the resonator is expediently designed in such a way that three modes are generated in the resonator. In a central measurement area, the electrical fields of the modes preferably point in different, expediently linearly independent, spatial directions, the spatial directions preferably being perpendicular to one another. The three modes are also decoupled. In the decoupling device belonging to a mode and leading to the respective detector, only resonance oscillations are detected which belong to this mode, with other modes oscillating independently of this having no part in this measurement signal.
  • In an alternative embodiment, the high-frequency resonator is designed as a forked resonator with an upper resonator part and a lower resonator part.A gap for the dielectric object to be measured is located between the resonator upper part and the resonator lower part. In the configuration with a fork resonator, flat material to be measured with anisotropic material properties, such as paper, Asian instant noodles (Yum Yum), wooden panels and non-metallic materials rolled in one direction (e.g. pasta) are measured with regard to their moisture and density. The two-dimensional objects to be measured are fed to the microwave measurement through the gap of the fork resonator, whereby the measurement can take place with the object in motion or with the object at rest. The material properties to be measured can be determined independently of the orientation of the material to be measured in the resonator.
  • Two modes are preferably generated in the fork resonator which reflect the flat properties of the material to be measured. The electric field lines of the measuring field in the measuring gap are preferably aligned parallel to the two-dimensional extent of the material to be measured. In the embodiment of the device according to the invention, two different resonance modes are used, the field lines of which are oriented perpendicular to one another and at the same time run parallel to the two-dimensional extent of the material to be measured.
  • In a preferred embodiment, the modes of the resonator are in a frequency range of 0.5 GHz and 20 GHz.
  • The geometric dimensions of the resonator, in particular the interior of the resonator, can have different shapes. In a preferred embodiment, the resonator interior is designed as a cuboid. Alternatively, the resonator can also have an interior space in the form of an ellipsoid or a cylinder shape with an elliptical cross section.
  • In order to be able to feed the electrical objects for measurement in the measuring range of the resonator, the resonator is preferably provided with an inlet and an outlet opening. The openings are arranged in the resonator in such a way that they allow movement of the dielectric object to be measured through the resonator. The movement can be a free movement in which the object to be measured falls freely through the resonator, for example under the influence of gravity. It is also possible to move the object to be measured freely through the resonator in an air flow.
  • In a further development, a format part for guiding the object to be measured is provided between the inlet and outlet openings. For example, the object to be measured can slide or fall freely through the resonator along the format part. This product-carrying, non-metallic, circular or rectangular tube has a cross-sectional diameter that is so large that all test specimens in question fit through, so the size of the test specimen is irrelevant. In a preferred embodiment, however, it is also possible, in order to restrict the possibilities of movement of the specimen during the movement through the resonator, to adapt the diameter of the specimen tube to the format of the specimen. In this case, when changing the format of the product, the format of the sample tube must also be changed. It is particularly advantageous to use format parts when measuring with two independent modes.
  • The object according to the invention is also achieved by a method for measuring the mass and / or moisture of dielectric objects independently of one another. In the method according to the invention, an evaluation unit is provided, at least one high-frequency generator and at least one high-frequency detector, which interact with a resonator in order to detect the change in a mode in the resonator. During the measurement process, at least two mutually decoupled modes are generated in the resonator and frequencies occurring in the resonator are measured with the at least one high-frequency detector for each of the modes. The decoupled modes are independent of one another and preferably form a measurement area in the resonator in which the electric fields point in linearly independent spatial directions. The evaluation unit determines a shift in the resonance frequency for the measured frequencies of each mode and evaluates a change in the resonance curve. The mass and / or moisture of the dielectric object is determined independently of one another from the shift in the resonance frequency and the change in the resonance curve.
  • The evaluation unit preferably evaluates a broadening of the resonance curve as a change in the resonance curve.
  • In an alternative method, the evaluation unit evaluates a change in the resonance amplitude as a change in the resonance curve.
  • In a preferred embodiment of the method, the at least one high-frequency detector determines the frequencies occurring in the resonator several times during a measurement cycle.
  • In the method according to the invention, each mode is preferably generated with a resonance frequency that is different from the resonance frequencies of the other modes. The resonance frequencies of the modes preferably each have a minimum frequency spacing in pairs. The minimum frequency spacing, which is at least 100 MHz, for example, ensures that the resonance modes can be evaluated individually and independently of one another. In this way, it is possible to determine the dielectric properties of the dielectric objects to be measured for each direction of a mode independently of the directions of the other modes, so that a precise analysis of the measurement results is possible and the results need not be averaged.
  • During the evaluation, the evaluation unit evaluates the change in the resonance curve and the shift in the resonance frequency for each mode, preferably at the same time.
  • The requirement for simultaneous evaluation, which can be made for rapidly moving specimens, requires the independent use of three microwave generators and three microwave detectors. If the movement of the specimen is slow enough, the measurement of the mutually independent resonance modes, taking place one after the other, can also take place by means of a switch of microwave generators and / or detectors to the different resonance modes.
  • In an advantageous embodiment of the method, the change in the resonance curve and the shift in the resonance frequency are evaluated for each mode at a point in time which is independent of the points in time at which the values ​​of the other modes are evaluated. This occurs in particular when the movement of the product through the resonator is controlled by a format-dependent guide tube.
  • Different approaches are also possible with regard to the generation of the resonances in the resonator. In a first approach, each mode is generated by exactly one high-frequency generator and measured by exactly one high-frequency detector. Alternatively, it is also possible to generate at least two modes using a high-frequency generator. As an alternative or in addition, it is also possible to measure the frequencies occurring in at least two modes by precisely one high-frequency detector.
  • A decoupling device is preferably provided for each mode and is located in a vibration node of the other modes. The resonance frequencies of the different modes are expediently drawn so far apart by special structural measures of the resonator that the separation of the modes can be amplified by special bandpass filters in each individual detector line.
  • In an alternative embodiment of the method according to the invention, the at least one high-frequency detector and / or the at least one high-frequency generator is switched between the modes.
  • In the method according to the invention, a phase shifter can also be used to generate a rotating electric field in the resonator via two high-frequency generators, especially if the two resonance oscillations to be superimposed are very close in frequency and thus a common rotating resonance is possible. The measurement of the common resonance is then a superposition of the two original resonances.
  • Three modes are preferably generated in the resonator. The modes are preferably oriented in the resonator in such a way that the electrical fields of the modes have different directions in a measurement area. The electric fields in the measurement area preferably point in linearly independent directions and are preferably perpendicular to one another.
  • In the method according to the invention, in particular for measuring flat material to be measured, a fork resonator with an upper resonator part and a lower resonator part can be used, with a gap being provided between the upper resonator part and the lower resonator part through which the dielectric object to be measured is guided. It is thus possible to arrange the dielectric object to be measured in the gap in a stationary or moving manner. In order to measure a flat material to be measured with anisotropic material properties regardless of its orientation in the resonator, the fork resonator preferably generates two modes with different resonance frequencies.
  • In order to achieve the highest possible throughput of objects to be measured, these are moved by the resonator during the measurement process. Here it is possible for the objects to move freely through the measuring area. Free movement can be generated, for example, by gravity in the case of objects falling freely or sliding on slides and / or by an air flow. In an alternative embodiment, the objects to be measured are moved through the measuring area in a guided manner. This means that the objects to be measured slide, for example, through the measuring area in format-dependent tubes.
  • A preferred embodiment is explained in more detail below. It shows:
  • Fig. 1
    Measurement results for different types of tablets with a uniform calibration of the measuring device, which move on a slide with a 40 ° incline through the microwave resonator,
    Fig. 2
    Measurement results for different tablets that move on a 40 ° slide through the resonator with a uniform calibration of the measuring device,
    Fig. 3
    a resonator with three independent resonance modes, and
    Fig. 4
    a fork resonator in a schematic representation for measuring flat goods.
  • A quick and very precise measurement of the mass of relatively small objects is particularly important in the area of ​​pharmaceutical products, such as tablets or capsules, since the total mass of these objects is proportional to their active ingredient content. Additional important information about the properties of these objects can be obtained from a measurement of the moisture, for example the mechanical properties of pressed tablets or hard gelatine capsules are heavily dependent on their moisture content and the capsule content also shows different properties at different moisture levels. A particular difficulty when measuring compressed tablets is that they can have any spatial shape. For example, the objects to be measured can be spherical, cube-shaped, elongated, triangular or otherwise shaped. For measurement objects that do not have spherical symmetry, the measurement in a resonator is always position-dependent. The result depends on the ratio of the surface portion of the object that is parallel to the field lines of the resonator to the surface portion of the object on which the field lines are perpendicular. If field lines and surface parts of the measurement object are parallel, the electric field is continuously converted into the product, the resonance frequency detuning of the resonator results from the effect of shortening the wavelength within the measurement product. When the resonance frequency is lowered to the new value, the field has almost the same spatial profile as in the case of the empty resonator. In this case, the perturbation-theoretical solution of Maxwell's equation gives the following expression for the relative change in the resonance frequency if the field profile does not change due to a product present:
    where f0 the resonance frequency of the empty resonator, fp the resonance frequency of the filled resonator standing for the effect of the parallel surface parts of the test object, F the ratio of the microwave field energy in the area of ​​the sample to that of the entire resonator and ε the relative dielectric constant (real part) of the sample material.
  • In contrast, if the field is oriented perpendicular to the surface portion of the measurement object, the electric field makes a jump at the transition into the sample material by the factor ε to the value E / ε. If one denotes with fs the resonance frequency, which is set in the filled resonator by the effect of the vertical surface components of the measurement object, then perturbation theory provides the expression for the relative frequency shift with vertical field orientation in the following expression:
  • A comparison of the two expressions makes it clear that an arbitrarily shaped measurement object that is located in the microwave resonator causes different measurement signals depending on its orientation to the microwave field in the measurement area. This is particularly important when measuring moving objects which, with the same volume, can pass the measuring field in different orientations and thus cause different microwave measurement values. This results in a limited accuracy of the mass and / or moisture measurement. Rolling or tumbling motion of the measuring object when passing through the measuring object also lead to measurement errors.
  • In a preferred embodiment of the method according to the invention, for the rapid mass and / or moisture measurement of arbitrarily shaped objects in any position, a microwave resonator is used using three simultaneously excitable resonance modes. In this exemplary embodiment, the electric fields of these resonance modes are perpendicular to one another and form an orthogonal tripod. It is thus possible to record the respective volume fractions of the measurement object in all three spatial directions with the same orientation of the electric field.
  • shows in principle the measurement setup of a resonator 12 with three independent, mutually perpendicular resonance modes. The resonance modes are generated by three high-frequency generators 14, 16, 18 and are coupled in on different sides of the resonator 12. The coupling of the respective resonance modes is shown schematically by arrows 20, 22 and 24. The feed line to band filters 26, 28, 30 is also shown schematically by arrows. As can be seen in the schematic diagram in FIG. 4, the high-frequency generator 16 feeds its signal on one flat side of the resonator 12, which is coupled out on the opposite flat side and passed on to the band filter 28 from where the filtered results are passed to detector 34. The same applies to the other spatial directions, with the generator 14 being coupled in on one flat side of the resonator 12 and the received signals being coupled out on the opposite side and being passed on to the bandpass filter 26. The filtered values ​​arrive at the detector 32. The procedure for the third spatial direction is analogous, with the signals from the generator 18 being coupled in on the large flat side of the resonator 12 and decoupled on the opposite large flat side, from where they are sent via the band filter 30 to the Detector 36 are forwarded.
  • A guide tube 38 through which the tablets 40 to be measured can fall freely is also shown schematically in FIG. The guide tube consists of a non-metallic material and leads through the center of the resonator 12. As indicated in FIG. 3 by the tripod 42, the electrical fields of the modes are perpendicular to one another in the center of the resonator 12. The functions of the band filters 26, 28, 30 are to filter the measured resonance values ​​and to attenuate or suppress undesired contributions to the resonances.
  • A prerequisite for the use of band filters is, of course, that the three high-frequency generators 14, 16, 18 work with resonance frequencies that are spaced apart from one another.
  • The data recorded by the detectors 32, 34, 36 are forwarded to an evaluation unit 44. The evaluation unit 44 is also connected to the generators 14, 16, 18 in order to control them.
  • The evaluation unit 44 then calculates the result of the mass 46 of the dielectric body 6 and the moisture 48 of the dielectric body. In general, moisture is always understood to be a concentration value that results from the quotient of the water mass and the total mass (dry mass) of a test specimen, given in percent.
  • A resonator with three mutually perpendicular resonance modes that can be excited at the same time can be produced in different designs. One possible embodiment consists in a resonator which is shaped like an ellipsoid. Another possible embodiment is the shape of a cuboid. It is also possible to provide a resonator which has the shape of a cylinder, the cross-sectional area of ​​which has the shape of an ellipse. The coupling of the mode into the resonator takes place in such a way that it is ensured for each of the three electric fields that only the desired resonance mode is excited. Furthermore, it must be ensured that no crosstalk occurs between the individual modes, which would falsify the measurement results. In the case of crosstalk, the phenomenon occurs that, when one mode is excited, additional further modes are excited, which make a contribution to the measurement result. Such crosstalk can be avoided on the one hand by a special arrangement of the field couplings, in that the coupling antennas or coupling apertures of the excited resonance mode are attached in the electrical node of the other resonance modes.On the other hand, structural measures on the resonator can ensure that the three resonance frequencies are so far apart that the other modes cannot interfere with the actual resonance mode through specially adapted bandpass filters.
  • In the preferred exemplary embodiment for the measuring method, the change in the resonance curve upon interaction with the material to be measured is recorded as the measured value for each resonance mode. Furthermore, the shift in the resonance frequency (A) is recorded for each mode. In the exemplary embodiment, the broadening of the resonance curve (B) is measured as a change in the resonance curve. The measuring process delivers a total of three measured values, so that the measured values ​​A1, A2, A3, B1, B2, B3 are available for each measurement object. These measured values ​​depend on the humidity and the mass of the measurement object in the three spatial directions that are assigned to the resonance modes. The mass and humidity of the entire measuring object can be calculated from the six measured values. The calculated values ​​for mass and moisture are then independent of the orientation of the measurement object in the measurement field and independent of the sample shape of the measurement object. If the measurement object is oriented differently in the measurement field, the signals vary in the individual spatial directions, but the measured mass and the humidity of the entire measurement object remain unaffected by the orientation.
  • The calculation of the mass and the moisture of the entire measuring object from the six individual measured values ​​(A.1, A2, A3, B1, B2, B3) is carried out as follows:
    where the coefficients kj (j = 1,, .., 7) and cj (j = 1, ..., 4) represent the calibration coefficients. The mass measurement is moisture-compensated, the moisture measurement obtained in this way is also independent of the mass of the measuring object.