Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Reexamination Certificate
2001-11-19
2004-06-15
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
Reexamination Certificate
active
06750657
ABSTRACT:
TECHNICAL FIELD
The invention relates to a combination of a feedthrough element for an electric high-frequency signal and a probe for guiding said high-frequency signal—as it is, for example, generated in a level metering device and evaluated after reflection at a filling product surface to be monitored—to the filling product surface and back from there. The feedthrough element comprises a guiding element, into which the electric high-frequency signal is to be fed at an inlet point, and which transmits the electric high-frequency signal to the probe at an outlet point. Moreover, the feedthrough element comprises a one-part or multipart mechanical carrier element. A one-part or multipart insulation is present between the carrier element and the guiding element. The invention further relates to level metering devices (TDR devices) working on the principle of transit or propagation time measurement of guided electromagnetic waves and being equipped with a feedthrough of the aforementioned type.
PRIOR ART
For level metering operations, measurement systems are used, which determine the distance from the filling product on the basis of the measured transit time of electromagnetic waves from a level metering device mounted in the receptacle cover to the surface of the filling product and back. The required level can be calculated when the receptacle height is known. Such sensors known under the technical designation of level radar, are all based on the property of electromagnetic waves of propagating within a homogenous non-guiding medium at a constant speed, and of being at least in part reflected at the boundary surface of various media. Each boundary layer of two media having various dielectric constants, generates a radar echo upon impingement of the wave. The greater the difference between the two dielectric constants, the more the impedance level of the wave propagation changes, and the stronger is the echo to be observed.
Various radar principles are known for determining the required wave propagation time. The two mainly used methods are the pulse-time delay method (pulse radar), for one, and the frequency-modulated continuous wave method (FMCW radar), for another. The pulse radar uses the pulse-shaped amplitude modulation of the wave to be emitted, and assesses the direct time interval between emission and reception of the pulses. The FMCW radar determines the transit time in an indirect way by emitting a frequency-modulated signal and by differentiating between emitted and received instantaneous frequency.
Apart from the various radar principles, various frequency ranges of the electromagnetic waves are used, as well, depending on the respective application. Thus, for example, pulse radars exist having carrier frequencies in the range from 5 to 30 GHz, and in addition likewise those working in the base band as so-called monopulse radar without carrier frequency.
A series of methods and devices is moreover known for guiding the electromagnetic wave to the surface of the filling product and back. Thereby, the basic difference is made between a wave radiated into the space and a wave guided through a line. A level measuring apparatus in which microwaves are fed via a coaxial line into an antenna meant for radiating electromagnetic waves is known from EP 0 834 722 A2. Here, the antenna is configured in two parts. One antenna part in the form of a solid cylinder consists of a dielectric material and is shrouded by a metal sleeve. The microwave is fed in at one end of the solid cylinder of a dielectric material, while at the other end ensues the transmission to the radiating end of the antenna. The metal sleeve extends over the antenna zone configured as a solid cylinder and being present in the zone of a neck of a vessel containing the filling product. This antenna structure, in particular the configuration within the neck of the vessel, therewith constitutes a filled waveguide for transferring the high-frequency signal or the wave into the antenna zone meant for radiation. This structure has the effect that the antenna, in the zone of the attachment of the measuring apparatus—hence in that part of the antenna situated in the zone of the neck—does not transmit microwaves and does not receive reflected microwaves, respectively. To avoid an impedance leap at that end of the metal sleeve facing the radiating antenna, the sleeve end is bevelled.
From EP 0 922 942 A1, a filling level measuring device with a radiating antenna is likewise known to work with microwaves. Here, the microwave fed through a coaxial cable, is introduced into an end element, which is configured with a cone at the antenna side. Following same, there is an insert of a dielectric material comprising a recess in the end element corresponding to said cone. Then from this insert of dielectric material ensues the further transmission of the microwave to the radiating antenna parts. A higher portion of ceramic is featured in the direction facing away from the antenna than in a section arranged in the transmitting direction facing the antenna in order to achieve a quasi-continuous transition without having substantial impedance leaps.
Radar sensors exhibiting a completely different structure with respect to the feedthrough and the signal guidance, which guide the electromagnetic wave through a line (probe) to the reflection place and back, are also designated as TDR (time domain reflectometry) sensors. These sensors, as compared to those which freely radiate high-frequency waves, have a substantially lower attenuation of the reflected echo signal, since the power-flow only ensues in the constricted area in the environment along the conducting waveguide. Moreover, interfering echoes from within the receptacle, originating, for example, from the reflections of the wave at receptacle components (stirrers, tubes), and which complicate the identification of the very one echo from the surface of the filling product with freely radiating sensors, are avoided to a large extent with sensors having guided waves. This leads to the fact that level metering with guided electromagnetic waves is to a large extent independent of the receptacle construction and moreover of the product properties of the filling product or other operational conditions (e.g. dust, angle of the bulk good), and therefore leads to highly reliable measurement results.
All known leads usual for high frequency can be used as the waveguides for guiding the wave, in which the wave penetrates at least in part the medium surrounding the metallic leads or is enclosed by same. Due to their simple mechanical structure and their suitability for any filling products, i.e. bulk goods and liquids, the single-wire line or single-wire probe in particular is often used in the level metering technology. In its configuration as a rod or cable probe, it is above all insensitive to deposits and adherences of filling products. In DE 44 04 745 C2, a level metering sensor including such a probe is described as an example.
An important aspect of the TDR level metering sensors having single leads, is the input of the measurement signal from the electronic unit into the probe. Thereby, it is important that the path leading from the electronic unit to the probe, does not contain any major impedance leaps for the guided wave. Since a part of the wave is reflected by every discontinuously changing line impedance, this reflected portion, for one, is no longer available for the measurement purpose, hence the reflection at the surface of the filling product, thus causing an amplitude loss of the echo generated there. Moreover, additional interfering echoes are generated by the wave reflecting at possible line impedance variations between the electronic unit and the probe, which complicate the identification of the filling product reflection to be evaluated. This is due, in particular, to the fact that the echo interfering at the irregularity between the receptacle feedthrough and the probe extends in each case depending on the bandwidth used of the measurement signal over a distance area
Fehrenbach Josef
Griessbaum Karl
Motzer Jurgen
Elise Timothy
He Amy
Le N.
Schwegman Lundberg Woessner & Kluth P.A.
VEGA Grieshaber KG
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