Measuring and testing – Liquid level or depth gauge
Reexamination Certificate
2000-07-11
2001-08-21
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
C343S872000, C343S888000, C343S900000
Reexamination Certificate
active
06276199
ABSTRACT:
The invention relates to a filling level measuring device operating with microwaves and to be fastened on a container, having a housing and a rod-shaped antenna which points into the container and consists of a dielectric, in particular of polytetrafluoroethylene (PTFE) or of polyphenylenesulfide (PPS).
In filling level measurement, microwaves are sent by means of an antenna to the surface of a filled substance and the echo waves reflected at the surface are received. An echo function representing the echo amplitudes as a function of the distance is formed and used to determine the probable useful echo and its delay time. The delay time is used to determine the distance between the surface of the filled substance and the antenna.
All known methods which make it possible to measure relatively short distances by means of reflected microwaves can be used. The most well known examples are pulsed radar and frequency-modulation continuous-wave radar (FMCW radar).
In the case of pulsed radar, short microwave transmission pulses, referred to in the following as wave packets, are transmitted periodically, reflected by the surface of the filled substance and received again after a distance-dependent delay time. The received signal amplitude as a function of time represents the echo function. Each value of this echo function corresponds to the amplitude of an echo reflected at a particular distance from the antenna.
In the case of the FMCW method, a continuous microwave which is periodically frequency-modulated linearly, for example on the basis of a sawtooth function, is transmitted. The frequency of the received echo signal therefore has with respect to the instantaneous frequency which the transmitted signal has at the instant of reception a frequency difference which depends on the delay time of the echo signal. The frequency difference between transmitted signal and received signal, which can be obtained by mixing the two signals and evaluation of the Fourier spectrum of the mixed signal, consequently corresponds to the distance of the reflecting surface from the antenna. Furthermore, the amplitudes of the spectral lines of the frequency spectrum obtained by Fourier transformation correspond to the echo amplitudes. Therefore, in this case, this Fourier spectrum represents the echo function.
Filling level measuring devices operating with microwaves are used in very many branches of industry, for example in chemistry or in the food industry. Typically, the filling level in a container is to be measured. These containers usually have an opening, at which a connection piece or a flange is provided for the fastening of measuring devices.
Rod-shaped antennas, frequently also referred to as a rod antenna or dielectric rod radiator, are always used with preference if the container opening has a small diameter. Rod antennas have a small external diameter in comparison with horn radiators.
In DE-A 44 05 855 there is described a filling level measuring device operating with microwaves and to be fastened on a container, having a housing and having a rod-shaped antenna which is fastened in the housing, points into the container and consists of a dielectric.
In DE-A 44 43 055 there is described a filling level measuring device operating with microwaves, in which metallic elements are arranged at a distance from one another on a rod-shaped antenna made of a dielectric. These elements are connected electrically conductively to one another. These metallic elements serve on the one hand for focusing the microwaves, and on the other hand for preventing static charging of the dielectric rod antenna.
A disadvantage of the abovementioned filling level measuring devices operating with microwaves is that a rod antenna both transmits microwave radiation substantially forward, as a main lobe into the interior space of the container and, on account of its directional characteristic, radiates it perpendicularly with respect to the antenna axis in the form of minor lobes. Typically, the amplitude of a first minor lobe is only 20 dB to 25 dB below the amplitude of the main lobe. Although the focusing by means of metallic elements described in DE-A 44 43 055 brings about an improvement here, it is not suitable for preventing radial radiation.
The radially transmitted component of the microwave radiation is reflected, at least partially, at the rim of the opening of the container and at the measuring device fastening, and therefore constitutes spurious radiation, which may impair the measurement considerably. This spurious radiation occurs even if the height of the measuring device fastening is very low, for example only a few centimeters. The spurious radiation component depends on the method of installing the measuring device and is particularly great, for example, in the case of measuring devices fastened on tubular connection pieces.
This spurious radiation is produced, for example, by a component of the transmitted microwave energy being reflected directly at the connection piece and subsequently received by the antenna, or going into the container, or by multiple reflections occurring in the connection piece. These reflections or multiple reflections cause an artificial lengthening of short microwave pulses to be transmitted. Filling level measurement by the pulsed radar method can only be carried out, however, if the delay time of the measuring signal is greater than the duration of the transmission pulse and the time in which the spurious signals just mentioned have decayed. For the FMCW method, it applies analogously that the frequency shift of the measuring signal must be greater than the frequency shift of the spurious signals.
The smaller the distance between the measuring device fastening, for example connection piece, and the antenna, the greater the effects are for the propagation of the microwaves in the near field of the antenna.
Apart from the microwave radiation used for the filling level measurement, which is transmitted directly from the antenna to the filled substance and is reflected there directly to the antenna, spurious radiation, which has been diverted by at least one reflection at the measuring device fastening, also reaches the antenna. This causes an erroneous determination of the delay time, and consequently erroneous measurement results.
A further disadvantage of the abovementioned filling level measuring devices is that condensate or moisture, for example, can cause a direct electrical connection to develop between the antenna and the container fastening. Such a short-circuit, just like the short-circuiting of a coaxial line, causes a reflection of the entire microwave energy to the transmitter and prevents the transmission of microwaves.
Even if such a short-circuit does not occur, condensate and/or moisture have very disadvantageous effects, since they have a direct influence on the propagation and reflection behavior of microwaves. For example, spurious radiation which occurred only to a small extent directly after the installation of the device may increase so much due to condensate and/or moisture that filling level measurement is no longer possible. This is particularly disadvantageous, since the operator of the device assumes that he has a measuring device which operates reliably.
It is therefore an object of the invention to specify a filling level measuring device operating with microwaves in which the antenna does not transmit any microwaves in the region of the measuring device fastening and in which it does not receive any microwaves reflected in the region of the measuring device fastening.
Furthermore, it is an object of the invention to specify a measuring device in which the transmitted power is unimpaired by condensate or moisture.
For this purpose, according to a first solution variant, the invention comprises a filling level measuring device operating with microwaves and to be fastened onto a container, having
a housing and
a rod-shaped antenna which is fastened in the housing,
points into the container and
consists of a dielectric, in particular of polytetrafl
Eckert Manfred
Oberle Klaus-Peter
Bose McKinney & Evans LLP
Cygan Michael
Endress + Hauser GmbH + Co.
Williams Hezron
LandOfFree
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