Patent Application
20140333470
The invention relates to an apparatus for determining or monitoring fill level of a fill substance in a container as defined in the preamble of claim 1.
In fill level measurement, microwaves are transmitted by means of an antenna to the surface of a fill substance and the echo waves reflected on the surface are received. The echo waves are presented as an echo function, from which travel time is determined. From the travel time, the separation between the surface of the fill substance and the antenna is determined.
All known methods can be applied, which enable measurement of relatively short distances by means of reflected microwaves. The best known examples are pulse radar and frequency modulation continuous wave radar (FMCW radar).
In pulse radar, short microwave transmission pulses, referred to in the following as waves, are periodically transmitted. These are reflected from the surface of the fill substance and received back after a distance dependent travel time. The received signal amplitude as a function of time is referred to as the echo function. Each value of this echo function corresponds to the amplitude of an echo reflected at a certain separation from the antenna.
In the FMCW-method, a continuous microwave is transmitted, which is periodically linearly frequency modulated, for example, according to a sawtooth function. The frequency of the received echo signal has, consequently, compared with the instantaneous frequency, which the transmission signal has at the point in time of the receipt, a frequency difference, which depends on the travel time of the echo signal. The frequency difference between transmission signal and received signal, which can be won by mixing the two signals and evaluating the Fourier spectrum of the mixed signal, corresponds, thus, to the separation of the surface of the fill substance from the antenna. Furthermore, the amplitudes of the spectral lines of the frequency spectrum won by the Fourier transformation correspond to the echo amplitudes. This Fourier spectrum is, consequently, in this case, the echo function.
Fill level measuring devices working with microwaves are applied in many branches of industry, e.g. in the chemicals industry and in the foods industry. Typically, it is the fill level in a container that is measured. These containers usually have an opening, where a nozzle or a flange is provided for securement of measuring devices.
Depending on application, fill level measuring technology usually involves use of parabolic-, horn- rod- or patch antennas. Horn antennas are basically so constructed that a funnel shaped metal horn is formed on a hollow conductor in the fill substance facing direction. The construction of a parabolic antenna can be described in simplified manner in the following way: the microwaves are guided in a hollow conductor, radiated directly, or by means of a reflector, and/or coupled back in the focal point of the parabolic mirror. A rod antenna is composed basically of a hollow conductor, which is filled at least partially with a rod of a dielectric and has a coupling structure in the form of a taper or cone facing in the direction of the fill substance. These three freely radiating antenna types are usually fed via a coaxial line, which is connected to an exciter element protruding into the hollow conductor.
A helical antenna is a helically shaped antenna for transmitting and receiving circularly polarized electromagnetic waves. The helical antenna is composed, in the case of unsymmetric (coaxial) supply, of one, or, in the case of symmetric supply, of two, conductors (band or wire) coiled into the shape of a screw.
The coil antennas likewise partially referred to as helical antennas are composed completely or partially of a single ply cylindrical coil, which has, however, dimensions, which are small compared with the wavelength. These antennas are, in principle, shortened quarter wave dipoles.
The winding direction of the helical antenna determines the direction of rotation of the radiated wave. Analogously, in the case of a helical antenna, those electromagnetic waves are received with the least loss, which have the same direction of rotation, as the winding direction of the helical antenna. Waves, which have another direction of rotation than the winding direction of the helical antenna, are, in contrast, received strongly suppressed. A helical antenna is able to receive waves linearly polarized in any direction. Therefore, they are often applied also in cases, where waves of undefined linear polarization are to be received.
EP 2 060 883 A1 describes a fill-level sensor, which has a first antenna for transmitting a transmission signal to a surface of the fill substance and a second antenna for receiving a signal reflected from the surface of the fill substance. Furthermore, the fill-level sensor includes a housing, which serves as outer shell for accommodating the first and second antennas. Furthermore, the housing has a cylindrical or conical external form, wherein the first and second antennas are embodied as horn antennas.
The known fill-level sensor receives the transmitted electromagnetic waves without paying attention to where they were reflected. The echo waves can be from the surface of the fill substance or a wall of the container or from interfering features, such as stirring mechanisms or the like.
An object of the invention is to provide a fill-level sensor, which ascertains a dependable value of fill level.
This object is achieved by the subject matter of the invention. The subject matter of the invention concerns an apparatus for determining or monitoring fill level of a fill substance in a container, comprising: at least two antennas, wherein a first antenna transmits electromagnetic waves in the direction of the surface of the fill substance and a second antenna receives reflected waves; and at least one evaluation unit, which ascertains fill level in the container based on travel-time difference of transmitted and reflected electromagnetic waves,
characterized in that the antennas are helical antennas, in order to transmit, respectively to receive, circularly polarized electromagnetic waves and that the evaluation unit detects a rotational direction change between the transmitted wave and the reflected wave.
If a circularly polarized wave is reflected only on the surface of the fill substance, the direction of rotation of the wave changes. If a circularly polarized waves is reflected on the surface of the fill substance and on one additional object, such as the container wall or a stirrer, the direction of rotation of the wave changes two times and the wave has at the receiver the same direction of rotation as at the transmitter. This means that a change of the direction of rotation of the circularly polarized waves results in the case of an odd number of reflections and no change of the direction of rotation of the circularly polarized waves results in the case of an even number of reflections. As a result, the circularly polarized waves, which arrive at the receiver with the same direction of rotation as when they were transmitted from the transmitter, are not used for the travel-time measurement, because they were reflected on the surface of the fill substance and on at least one additional location. In this way, a part of the waves, which can corrupt the travel-time measurement, can be eliminated.
In a further development, two antennas are provided, wherein a first antenna is embodied as a transmitting antenna and a second antenna as a receiving antenna, wherein the first antenna has a polarization direction opposite to that of the second antenna. The opposite polarization direction is achieved by a respectively opposite winding direction of a helical antenna.
If the transmitting and receiving antennas have opposite winding directions, the receiving antenna only receives circularly polarized waves, which have an opposite direction of rotation relative to the transmitted, circularly polarized waves. Therefore, taken into consideration for the travel time determination are only the waves having an uneven number of reflections, while the disturbing, multiply times reflected waves are eliminated.
In a further development, three antennas are provided, wherein a first antenna is embodied as a transmitting antenna, and a second and a third antenna are embodied as receiving antennas, wherein the second antenna has a winding direction of the same sense as the first antenna and the third antenna has a winding direction opposite to that of the first antenna.
In an additional form of embodiment, three antennas are provided, wherein a first antenna is embodied as a receiving antenna and a second and a third antenna are embodied as transmitting antennas, wherein the second antenna has a winding direction of the same sense as the first antenna and the third antenna has a winding direction opposite to that of the first antenna.
In a further development, the windings of the antennas are conically embodied, especially they are cone shaped.
In a further development, the antennas are funnel shaped with two oppositely lying openings, and the electromagnetic waves exit from a first opening, which has a larger aperture area than a second opening.
In a further development, the antennas are funnel shaped with two oppositely lying openings, and the electromagnetic waves exit from a first opening, which has a smaller aperture area than a second opening.
In a further development, the antennas are at least partially filled with a dielectric, especially a synthetic material, e.g. a plastic.
In a further development, the antennas have a housing transmissive for electromagnetic waves.
In a further development, at least two of the antennas are isolated by means of a partition, so that electromagnetic waves of the two antennas do not superimpose within the housing.
The invention will now be explained in greater detail based on the appended drawing, the figures of which show as follows:
a three conical helix antennas in a dome, wherein the antennas are decoupled by means of three partitions,
b two conical helix antennas, which are decoupled by means of a partition,
a an embodiment of the apparatus of the invention with two mixers, wherein the intermediate frequency signals of the two mixers can be combined via a switch to form a total intermediate frequency signal or be selected sequentially in time,
b an embodiment of the apparatus of the invention with two mixers, wherein a partition, which is half the size of the antennas, isolates the antennas,
c an embodiment of the apparatus of the invention with two mixers, wherein a partition, which is the same size as the antennas, isolates the antennas.
The transmitting/receiving separator, or directional coupler, leads in the case of this embodiment as a directional coupler with unilaterally matched termination to a power loss of about 6 to 8 dB. With the application of a circulator, the power loss amounts to about 1 to 2 dB.
A concrete embodiment of the apparatus of the invention is shown in
If an electromagnetic wave is produced in the transmitting antenna 2, the electromagnetic wave leaves the transmitting antenna 2 as a circularly polarized wave due to the helical shape of the antenna. If the circularly polarized wave strikes the surface of the fill substance, this changes its direction of rotation. The reflected wave has, thus, an opposite direction of rotation as compared with the transmitted wave. The receiving antenna 4 has an opposite winding direction as compared with the transmitting antenna 2. Now the reflected wave has the same direction of rotation as the winding direction of the receiving antenna 4. As a result, the reflected wave is received by the receiving antenna 4 with especially low loss.
If, in contrast, the emitted wave is reflected on the surface of the fill substance and on an additional area, it has, after a double change of its direction of rotation, the same direction of rotation as the emitted wave. Since the reflected wave now has an opposite direction of rotation as the receiving antenna 4, the wave is received with especially high loss.
This allows the converse conclusion that an especially high loss receipt of the reflected wave must have an opposite direction of rotation as the winding direction of the receiving antenna 4 and an especially low loss receipt of the reflected wave must have the same direction of rotation as the winding direction of the receiving antenna 4.
Thus, the wave received with high loss has experienced an even number of reflections and the wave received with low loss has experienced an odd number of reflections. An even number of reflections shows that the wave was reflected on the surface of the fill substance and on at least one additional area.
Thus, the wave received with low loss is not taken into consideration for the travel time determination. In this way, waves corrupting the travel-time measurement can be eliminated. Due to the exponential decrease of amplitude upon each reflection, it can be ascertained which wave received with low loss has experienced only one reflection. Then only this wave is taken into consideration for travel time determination.
In the case of some waves, no one hundred percent change of the direction of rotation occurs upon reflection. Referenced to power, this is true for about 1% of the waves. This residue is received in the case of waves reflected with low loss.
The transmitting antenna 2 transmits a circularly polarized wave. If this wave experiences an odd number of reflections, the reflected wave reaches the dome 25 with an opposite direction of rotation. Since the first receiving antenna 4 has the same winding direction as the transmitting antenna 2, the wave is received by the first receiving antenna 4 after a one time reflection on a surface with an especially high loss. The second receiving antenna 5 has a winding direction opposite to that of the transmitting antenna 2. The wave is received by the second receiving antenna 5, consequently, with especially low loss. The electronic circuit can recognize such and uses this wave for travel-time measurement. Moreover, the difference between the signal of the first and second mixers 12, 13 can be taken into consideration for detection of the near range in the evaluation of an envelope curve.
If, in contrast, the transmitted wave is reflected on the surface of the fill substance and on an additional area, its direction of rotation does not change. This wave is received by the first receiving antenna 4 with low loss and by the second receiving antenna 5 with high loss and, consequently, is not taken into consideration by the electronic circuit for travel-time measurement.
a shows the three antennas 2, 4, 5 of the apparatus 1 illustrated in
b shows two antennas 2, 4 in a dome 25, such as they are arranged in the example of an embodiment corresponding to
A signal from the transmission oscillator 11 is switched between the first amplifier 14 and the second amplifier 15. An option, however, would be to provide separate oscillators for the two transmitting amplifiers. In the case of application of the amplifier as a switch, the reaction (scattering parameters) should be as small as possible.
By comparing the first and second signal paths, likewise certain signals, which are not evaluated, respectively taken into consideration, for travel time determination, can be eliminated.
a shows another form of embodiment of the apparatus 1 of the invention, which is constructed similarly to the apparatus 1 of
In this form of embodiment, a part of an exciter signal of the transmission oscillator 11 flows via the circulator 6 on a second signal path through the second receiving amplifier 18 and the second mixer 20. The signal of the second signal path is compared with a signal of the first signal path, which flows through the first receiving amplifier 17 and the first mixer 19 to the third amplifier 16 and the receiving oscillator 21. A switching between the first and the second signal path can occur by means of the first and second receiving amplifiers 17, 18. Comparison of the first and second signal paths allows identification of signals, which are not to be taken into consideration for travel time determination.
b shows another form of embodiment of the apparatus 1 of the invention according to
c shows another form of embodiment of the apparatus 1 of the invention according to
The partitions 28 serve the purpose of attenuating cross polarization and assuring that the electromagnetic waves, which the transmitting antenna 2 transmits, are not superimposed within the dome 25 with the electromagnetic waves, which the receiving antenna 4 receives.
Furthermore, an option is to measure in a container with such an apparatus using a reflector, for example, at an angle of 45°. The electromagnetic waves are rotated in the transmitting path as well as in the receiving path, in each case, once by 180° in the polarization direction. The relationship between direct and multiply reflected signals remains, however.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2013 104 699.1 | May 2013 | DE | national |
