Communications: directive radio wave systems and devices (e.g. – Determining distance – Material level within container
Reexamination Certificate
2002-01-18
2004-01-13
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Determining distance
Material level within container
C342S175000, C342S198000
Reexamination Certificate
active
06677891
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a filling level measuring device working on the principle of the transit time measurement of electromagnetic waves in the frequency range of below 3 GHz, in particular of below 2.5 GHz.
For the filling level measurement, measurement systems are used, which determine the distance between sensor and filling products on the basis of the measured transit time of electromagnetic waves from a sensor—in general also designated as filling level measuring device—mounted in a receptacle cover, to the surface of the filling products and back. With knowledge of the receptacle height, the required filling level can then be calculated. Such filling level measurement devices, also known under the technical term filling level radar, are all based on the property of electromagnetic waves of propagating within a homogenous, nonconductive material at a constant speed and of being reflected at least in part at the boundary surface of different media. Each boundary surface of two media having different dielectric constants generates a radar echo upon impingement of a wave. The larger the difference between the two dielectric constants, the more the wave resistance of the wave propagation alters, and the greater is the echo to be observed.
For determining the required wave transit time, various radar principles are known. The two principally used methods are, for one, the pulse transit time method (pulse radar), and, for another, the frequency-modulated continuous wave method (FMCW radar). The pulse radar uses the pulse-shaped amplitude modulation of the wave to be radiated, and determines the direct time interval between transmission and reception of the pulses. The FMCW radar determines the transit time in an indirect way by emitting a frequency-modulated signal and the differentiation between the emitted and the received instantaneous frequency.
Apart from the different radar principles, different frequency ranges of the electromagnetic waves are also used, depending on the application. Thus, pulse radars exist, for example, having carrier frequencies comprised between 5 and 30 GHz, and there exist others working in the base band as so-called monopulse radars without carrier frequency.
Moreover, a series of methods and devices are known guiding the electromagnetic wave to the filling product surface and back. Thereby, the basic difference is made between a wave radiated into the space, and a wave guided through a line. With waves radiated into the space, a differentiation into so-called bistatic arrangements and so-called monostatic arrangements is possible.
Bistatic arrangements have two separate antennas, one of these serving for emitting and the other for receiving. In monostatic arrangements, a single antenna serves at the same time for sending and receiving.
Sensors with guided electromagnetic waves working on the reflection principle, normally are always configured monostatic. An advantage of a monostatic sensor realization results in saving a second antenna or waveguide terminal, and therewith in a simple, space-saving structure. A disadvantage arises by the circuit-technical necessity of connecting the transmitter circuit, as well as the receiver circuit—these are also designated as transmitting and receiving means—with an antenna or a waveguide terminal connected upstream thereof. Thereby, the problem arises of mutually isolating the transmitter and receiver despite the common connection to the antenna and the waveguide terminal. Isolation means in this case the prevention of a signal change-over from the one circuit part to the other. Without this isolation, the transmitted signal would reach the receiver in the direct way, and would generate a comparatively large reception signal due to its relatively high amplitude.
Compared to the intended reception signals, which result from reflections on the measurement distance, and which are of a more or less high attenuation due to the transmission distance and the reflection at the measurement object, the transmitted signal, that reaches the receiver without isolation, causes a reception amplitude which is higher by a multiple. Despite the fact that this transmitted signal registered in the receiver arrives earlier from the measurement object than the echo to be evaluated, and that it is therewith not directly interfered with useful echoes, serious problems can arise due to the lacking isolation complicating or even preventing a distance measurement. Under these circumstances, the amplitude dynamics of the receiver must be adapted to a much higher range. In case the receiver cannot process the high amplitude of the transmitted signal, and even a limiting effect occurs, such an effect can remain effective for some time, until the time interval in which the useful echoes have to be expected. As a consequence thereof, the useful echoes would not be correctly receivable, and the measurement result would be questioned. In addition, it has to be expected that the impedance matching of the receiver is not ideal. A minor portion of each incoming echo signal is again reflected at the receiver input, and usually arrives at the receiver after a further reflection within the circuit, a second or third, etc., time. This phenomenon which can be characterized as ringing, plays no role in usual echo signals originating from reflections within the measurement distance. With a lacking isolation between transmitter and receiver, however, these signals of the transmitted signal, which have been reflected several times, are still in the order of useful echo amplitudes despite a corresponding attenuation. Due to their temporal position following the transmitted signal, they are interfered with reflections of measurement objects placed closed to the sensor. This leads to measurement errors in the proximity of the measurement device.
For interconnecting the sender, receiver and antenna or waveguide terminal, the following solutions are known. In the document DE 42 40 492, the use of a directional coupler or circulator is proposed. As to the practical circuit configuration of such a circuit part, no indications can be found therein. But it is described how a reception mixer can likewise be realized with directional couplers in such a way that the receptions signal and the local oscillator signal are mutually isolated. The directional couplers in this publication are realized as so-called hybriding couplers in microstrip technology. Similar directional coupler solutions can also be found in the U.S. Pat. No. 3,621,400.
The microstrip technology, however, is only applicable in a reasonable manner for signal frequencies in the microwave range (3-30 GHz). Usual radar sensors, however, use not only carrier frequencies, as has already been mentioned, e.g. in the range between 5 and 30 GHz, but there are also monopulse radars, which emit short pulses without using carrier frequencies. The frequency range of these transmitted signals typically is in the range of some Megahertz up to a few Gigahertz. Therewith, a directional coupler realization in microstrip technology is out of question for such monopulse radars.
In the document U.S. Pat. No. 5,517,198, a possibility is described, how in such a pulse radar—often also designated as TDR (time domain reflectometry)—a wide band isolation can be achieved between the transmitter and the receiver. The solution described therein uses a resistor bridge for dividing the transmitted signal into two identical portions. The one portion is directly fed into a branch of the receiver, the other one arrives in a second branch of the receiver, as well as in the measurement distance. Reflections from the measurement distance preponderantly arrive only in the second reception branch. By a differentiation between the two reception branches, the transmitted signal is eliminated from the receiver, while the reflections from the measurement distance remain uninfluenced. This solution is relatively extensive, since two reception branches are provided, and high amplitude attenuations of the useful
Fehrenbach Josef
Griessbaum Karl
Raffalt Felix
Schwegman Lundberg Woessner & Kluth P.A.
Sotomayor John B.
VEGA Grieshaber KG
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