Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Reexamination Certificate
1998-09-16
2001-11-13
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
Reexamination Certificate
active
06316946
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a microwave leakage field sensor for measuring moisture and/or density of dielectric materials.
2. Description of the Related Art
Both for application under conditions of a harsh industrial process and for serial measurement in the laboratory, microwave resonator technology affords the possibility of rapidly and precisely acquiring a statement about the moisture and density of the product to be investigated. What is essentially responsible for this is the particular method of separating the influence of moisture and density, which method can be employed with the use of resonators (EP 0 468 023).
In the resonator method, a standing microwave is generated in a suitable metallic cavity. An essential feature of this method consists in not just acquiring the no-loss effects of the interaction between the material moisture and the microwaves, that is to say all the property changes which are dominated by the real part of the dielectric constant. Rather the resonator method also enables the microwave power losses which influence the measurement result to be limited to the attenuation in the product (conversion into heat) and hence corruption due to other losses (leakage losses at surfaces or at coarse-grained scattering centers in the product, radiation losses, etc.) to be avoided. If a moist dielectric material is introduced into the resonator, then the resonant frequency is shifted and the width of the resonant curve increases. Density and moisture can be determined from these changes, as is explained in the abovementioned document.
The method presupposes the use of a resonator which is closed to a greater or lesser extent and from which no microwave energy can escape. Specifically, if field energy is lost by radiation, then this results in a corruption of the line width and of the resonant frequency, so that density and moisture can no longer be determined.
The limits in practical application of cavity resonators are imposed due to the fact that the product to be measured must be put into the sample tube of the applicator. In the case of many products, however, that is only possible in laboratory measurement if laboratory personnel fill the product manually into the sample applicator. In process measurements, in the case of products with good flowing or pouring behavior it is possible for a measurement to be effected in a bypass by transferring product from the main stream with the aid of conveying elements, filling it into the measurement tube and then conveying it back into the main stream again.
It is therefore desirable for process measurement technology to develop, as an expansion, a measuring device which has all the advantages of the patent, mentioned in the introduction, of a density-independent resonator measurement method for measuring moisture and, at the same time, can be directly incorporated in the main stream of the product flow without interfering with the process.
Although there are no difficulties at all in conducting microwaves from waveguides or cavities into the free space, so that the microwaves impinge on large-area plates, a stream of material in an industrial process, etc., in doing so an antenna-like radiation of microwave energy takes place, which, as already mentioned, effects the shifting of the resonant frequency and the widening of the resonant curve and thus corrupts the measurement result.
In the case of a previously known open leakage field resonator, the standing wave is generated by a coaxial line which is open on one side, resonator frequency and quality factor of the resonator being altered by a dielectric in the leakage field (Mesures Regulation Automatisme., Vol. 50, No. 1, January 1985, Paris FR, pages 67-70, XP002057631). The essential deficiency of this arrangement is that this form of leakage field has a distinct radiation behavior perpendicularly to the direction of the coaxial conductor. As a result, the essential precondition is not satisfied in order to be able to utilize the measurement of the losses and ot the resonant frequency shift for the purpose of separating moisture and material density. The layer thickness of the sample, the contact pressure between sensor and sample, the form of the sample, etc thus also have an interfering effect on the measurement signal.
A further previously known sensor with leakage field at the open line end of a coaxial conductor is intended to be used for measuring moisture (Thompson F: “Moisture Measurement Using Microwaves”, Measurement and Control, Vol. 22, No. 7, September 1989, pages 210-215, XP000052731). In order to gain a certain degree of control over the problem of radiation and hence of the diverse interfering influences (form of the sample, etc.), the leakage field is limited to “a few millimeters penetration depth into the sample” This also does not allow reliable and accurate measurements for measuring moisture and/or density of samples of relatively large dimensions.
SUMMARY OF THE INVENTION
The object consists in providing a leakage field sensor in the case of which, although the microwave field occurs in the space in front of a certain area, the undesirable antenna-like radiation virtually does not occur.
According to the invention, it was discovered from the inventors' calculations and experiments that this can be achieved by virtue of the fact that the microwave leakage field sensor is of essentially rotationally symmetrical design and transmissive for electromagnetic radiation in the axial direction toward at least one side, and that an essentially rotationally symmetrical alternating field of standing waves can be generated in it, the spatial period of which field in the peripheral direction is less than the vacuum wavelength at the frequency of the alternating field.
In other words, an essentially rotationally symmetrical electromagnetic alternating field in the form of a standing wave is generated, in the case of which the wavelength at the generation location is substantially less than in the free space. The field has a circumference and the distance along the circumference between points of equal field strength is substantially shorter than the wavelength of electromagnetic waves at the frequency of said field in a vacuum. As a result of this difference in the wavelengths at the location where the alternating field is generated and in the free space, the effect according to the invention occurs whereby although the microwaves penetrate a certain distance into the space, the microwave field strength decreases to a very great extent with distance from the leakage field sensor, with the result that virtually no microwave field strength is radiated. This is based essentially on the fact that extinction on account of interference occurs in the far field and only the near field remains.
The invention therefore provides a sensor which may also be of planar design, is designed, on the one hand, as a microwave resonator and is thus accessible to evaluation electronics based on the patent described. On the other hand, an electromagnetic field is generated above a planar sensor surface, which field projects into the product to be measured and has no radiation losses as long as the dielectric constant of the product lies below a critical limit value—which can easily be complied with in the practice of products occurring in an industrial context. Under these conditions, the losses measured by the planar resonator technique arise not as a result of radiation effects but rather as a result of the heat conversion within the product—as in the case of the cavity resonator. The measurement is thus possible from one side of large-area materials, such as, for example, wooden boards, masonry slabs, etc. The measurement is also possible by incorporation in container walls, silos or in the moving product stream within the process installations, etc.
An advantageous embodiment is distinguished by the fact that the leakage field sensor has a metal wire in the form of a closed conductor loop surrounded by a dielectric
Herrmann Rainer
Zaage Stefan
Alix Yale & Ristas, LLP
Leroux E P
Manfred Tews
Metjahic Safet
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