EP1325289A1

EP1325289A1 - Level meter

Info

Publication number: EP1325289A1
Application number: EP01965161A
Authority: EP
Inventors: Roland Müller; Wolfram LÜTKE
Original assignee: Endress and Hauser SE and Co KG
Current assignee: Endress and Hauser SE and Co KG
Priority date: 2000-10-10
Filing date: 2001-08-02
Publication date: 2003-07-09
Also published as: WO2002031450A1; US20020040596A1; AU2001285862A1; DE10049995A1; US6606904B2

Abstract

The invention relates to a level meter, which operates while using microwaves, for measuring the level of a filling material (5) in a container (1) and which requires only one antenna (7) of the simplest possible design, and in which a minimum distance of the filling material (5) from the antenna (7) required for conducting a measurement is as short as possible. The inventive level meter comprises a microwave generator (39) and an antenna (7) having a planar antenna structure, which serves to emit microwaves toward the filling material (5) and to receive microwaves reflected by a surface of the filling material. To this end, said planar structure has at least two emitting and/or receiving elements (11, 13, 33, 35, 37, S, E).

Description

level meter

The invention relates to a level measuring device working with microwaves for measuring a level of a filling material in a container with a microwave generator and an antenna with a planar antenna structure, which serves to transmit the microwaves in the direction of the filling material and to receive microwaves reflected on a surface of the filling material. In these level measuring devices, a transit time of the microwaves from the device to the product surface and back is usually determined by means of a reception and evaluation circuit, and the instantaneous level is determined therefrom.

In the level measurement, microwaves are sent to the surface of a medium by means of an antenna and echo waves reflected on the surface are received. An echo function representing the echo amplitudes as a function of the distance is formed, from which the probable useful echo and its transit time are determined. The distance between the product surface and the antenna is determined from the transit time.

The European patent application filed on September 7, 1999 with application number 99 11 7604.1 describes an antenna with a planar antenna structure that is suitable for level measurement.

Such planar antennas are also in the book 'Introduction to the theory and technology of planar microwave antennas in microstrip line technology' Gregor Gronau, Nellissen-Wolff bookstore or in the journal article 'Impedance of radiation slot in the ground plane of a microstrip line', IEEE Trans.Antennas Propagat ., Vol AP-30, pages 922-926, May 1982.

To determine the fill level, all known methods can be used which make it possible to measure relatively short distances using reflected microwaves. The best known examples are the pulse radar and the frequency modulation continuous wave radar (FMCW radar).

In the case of pulse radar, short microwave transmission pulses, referred to below as wave packets, are transmitted periodically, which are reflected by the product surface and be received again after a distance-dependent runtime. 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 certain distance 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 therefore has a frequency difference compared to the instantaneous frequency that the transmission signal has at the time of reception, which depends on the transit time of the echo signal. The frequency difference between the transmitted signal and the received signal, which can be obtained by mixing both signals and evaluating the Fourier spectrum of the mixed signal, thus 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. This Fourier spectrum therefore represents the echo function in this case.

The problem with level measurement with only one antenna arises that a meaningful level measurement is only possible if the level does not fall below a minimum distance from the antenna. This minimum distance, which is often also referred to as the block distance, is due to the fact that a received signal caused by the transmission must first have decayed to an amplitude below the echo amplitude before the echo signal reflected from the product surface can be reliably recognized and evaluated.

This problem can be largely solved by using two separate antennas, one for transmitting and one for receiving microwaves. This solution requires, however, that there are two openings on the container at a suitable distance through which the two antennas can be inserted. However, this is not the case for most applications.

EP-B 592 584 describes a level measuring device working with microwaves

- a microwave generator and

- an antenna, which serves to send the microwaves in the direction of the filling material and to receive microwaves reflected on a filling material surface,

- In which a transmitting element and a receiving element are arranged.

Crosstalk from the transmitter to the receiver is reduced here by generating linearly polarized microwaves in a first polarization plane and passing them through a phase shifter. The phase shifter is dimensioned so that the emerging microwaves e.g. are left circularly polarized. The right-hand circularly polarized microwaves are then received by the reflection on the product surface and converted into linearly polarized microwaves by means of the phase shifter. The polarization plane of these microwaves is perpendicular to the first polarization plane. The receiver is designed such that it only receives microwaves with this polarization, but does not pick up microwaves polarized along the first polarization plane.

However, such an antenna is very complex to manufacture because it requires appropriate filters and phase shifters. It is also relatively large and power is lost each time the microwaves pass through a filter or phase shifter.

It is an object of the invention to provide a level measuring device working with microwaves, which manages with a single antenna that is as simple as possible and in which a minimum distance of the filling material from the antenna required for measurement is as small as possible.

For this purpose, the invention consists in a level measuring device working with microwaves for measuring a level of a filling material in a container

- a microwave generator,

an antenna with a planar antenna structure,

which serves to send the microwaves in the direction of the filling material and to receive microwaves reflected on a filling material surface,

- In which the planar antenna structure has at least two transmitting and / or receiving elements. According to one embodiment, the transmitting and / or receiving elements are each located in a partial area of the antenna.

According to one embodiment, the transmitting and / or receiving elements are arranged nested in one another.

According to one embodiment, at least one of the transmitting and / or receiving elements is used exclusively as a receiver for measuring a fill level in a close range in front of the antenna.

According to a further development, all of the transmitting and / or receiving elements serve as transmitters and receivers for measuring a fill level in a remote area in front of the antenna.

According to a further development, at least one transmitting and / or receiving element serves exclusively as a transmitter and the other transmitting and / or receiving elements serve exclusively as a receiver, and a difference signal is determined which is the difference between the transmission signals applied to the transmitters and those received by the receivers Received signals corresponds.

According to an advantageous embodiment, the microwaves to be transmitted have frequencies that are greater than 20 GHz.

An advantage of the invention is that the antenna provides a very high degree of flexibility due to the planar antenna structure. The antenna structure can be divided at will into several transmitting and / or receiving elements and each transmitting and / or receiving element can be used optimally. As a result, a high transmission power is available, for example, for measurements in the long range, and for measurements in the close range, crosstalk from the transmitter to the receiver is significantly reduced by dividing the transmitting and / or receiving elements into pure transmitting elements and pure receiving elements. The only additional effort that is required to use these advantages is a corresponding wiring of the individual transmitting and / or receiving elements. This is easy to do and does not result in any loss of performance. The invention and further advantages will now be explained in more detail with reference to the figures of the drawing, in which four exemplary embodiments are shown; Identical parts are provided with the same reference symbols in the figures.

Fig. 1 shows schematically a measuring arrangement with a level measuring device arranged on a container and working with microwaves;

2 shows a schematic illustration of a planar antenna structure which has a first transmitting and / or receiving element and a second transmitting and / or receiving element arranged in another part of the antenna structure;

Fig. 3 shows a schematic representation of a planar antenna structure which has two transmission and / or reception elements arranged opposite one another, which are separated from one another by two further transmission and / or reception elements arranged opposite one another;

Fig. 4 shows a schematic representation of a planar antenna structure which has a plurality of transmitting and / or receiving elements arranged in a first antenna half and a plurality of transmitting and / or receiving elements arranged in a second antenna half;

5 shows a schematic representation of a planar antenna structure which has a plurality of transmission and / or reception elements arranged in a chessboard-like manner and nested in one another;

6 shows a basic circuit diagram of a fill level measuring device according to the invention, in which some transmitting and / or receiving elements operate as transmitters and some transmitting and / or receiving elements operate as receivers;

FIG. 7 shows a basic circuit diagram of a fill level measuring device according to the invention, in which a part of the transmitting and / or receiving elements serve exclusively as a transmitter and the other transmitting and / or receiving elements serve exclusively as a receiver for measurements in the close range, and all transmitters are used for measurements in the far range - and / or receive elements serve as transmitters and receivers; and 8 shows a basic circuit diagram of a fill level measuring device according to the invention, in which some transmit and / or receive elements work as transmitters and some transmit and / or receive elements act as receivers and a difference is formed from a transmit signal and receive signal, which are used to determine the fill level becomes.

Fig. 1 shows schematically a measuring arrangement with a level measuring device 3 arranged on a container 1 and working with microwaves. The level measuring device 3 is e.g. attached to the container 1 by means of a flange connection. The container 1 is filled with a filling material 5, the filling level of which is to be measured.

The fill level measuring device 3 comprises an antenna 7, with a planar antenna structure described in more detail below in connection with the exemplary embodiments shown in FIGS. 2 to 5. The antenna 7 projects into the container 1 and serves to send microwaves in the direction of the filling material 5 and to receive microwaves reflected on a filling material surface. For this purpose, the planar antenna structure has at least two transmitting and / or receiving elements. The fill level measuring device 3 has a microwave generator 9. A suitable microwave generator 9 is e.g. a pulse radar device constructed in planar circuit technology, an FMCW device constructed in planar circuit technology or a continuously oscillating microwave oscillator constructed in planar circuit technology.

The microwave generator 9 feeds those transmitting and / or receiving elements which either serve exclusively as transmitters or as transmitters and receivers.

Microwaves with frequencies above 20 GHz, for example 24 GHz, are preferably generated and transmitted. Above 20 GHz, the wavelength of the microwaves is sufficiently short to be able to use relatively small planar antenna structures. At 24 GHz, for example, several transmitting and / or receiving elements can be accommodated comfortably on an antenna structure with a diameter of a few centimeters to one or two decimeters. These small dimensions offer the advantage that the Antenna 7 is correspondingly small and can therefore also be easily inserted through container openings of small nominal width.

The microwaves are transmitted from the transmitting and / or receiving elements serving as transmitters to the product surface and the microwaves reflected on the product surface are received by the transmitting and / or receiving elements serving as receivers. The incoming microwaves are fed to a receiving and evaluation circuit 41, which determines the transit time of the microwaves to the product surface and back and determines the instantaneous fill level therefrom.

FIG. 2 schematically shows an exemplary embodiment of a planar antenna structure which has a first transmitting and / or receiving element 11, 13, shown by solid lines in FIG. 2, and a second, shown by dotted lines in FIG. 2. The transmitting and / or receiving elements 11, 13 are metallic structures which are applied to a side of a dielectric substrate 15 facing away from the filling material 5. On a side of the dielectric substrate facing the filling material, a conductive layer 17, for example in the form of a coating of the dielectric substrate 15, is provided, which has recesses 19, the shape and arrangement of which is preferably selected such that only electromagnetic waves of a desired mode are emitted , In the example shown, they are ^!

Recesses 19 slots extending radially outwards, which are particularly suitable for transmitting the TE-01 mode.

A protective layer 21, which consists of a dielectric, is provided on a side of the conductive layer 17 facing away from the dielectric substrate 15. This protective layer 21 forms the termination of the antenna in the direction facing the product. The dielectric is therefore preferably a chemically resistant material, e.g. Polytetrafluoroethylene (PTFE) or a composite material made of polytetrafluoroethylene and ceramic and / or glass fiber.

A further dielectric disk 23 is provided on a side of the substrate 15 facing away from the conductive layer 17 and in operation from the filling material 5. The disk 23 has a recess 25 through which a connection element (not shown in FIG. 2) can be inserted. The connection element must be designed so that it connects to each transmission and / or receiving element 11, 13. The electrical connection of the conductive layer 17 to ground or to a fixed reference potential is preferably also carried out via this connection element.

For this purpose, either through holes 27 can be provided in the substrate, through which appropriately shaped and arranged pins of the connecting element can be guided to the conductive layer 17, or contact pins connected to the conductive layer 17 can pass through the substrate 15 at the same location be led.

In the exemplary embodiment shown in FIG. 2, the first transmitting and / or receiving element 11 is arranged in a first part of the antenna structure and the second transmitting and / or receiving element 13 is arranged in a second part of the antenna structure. Each of the transmitting and / or receiving elements 11, 13 is a structure which extends radially outward in the manner of a branch and which in each case fills a semi-circle of the substrate 15. The branches of the first transmitting and / or receiving element 11 converge at a point 29 in the middle of the substrate, at which the first transmitting and / or receiving element 11 is connected to the connecting element. The branches of the second transmitting and / or receiving element 13 converge in a circular segment 31 near the center of the substrate, on which the second transmitting and / or receiving element 13 is connected to the connecting element.

3 shows a schematic representation of a further planar antenna structure. For the sake of simplicity, only the substrate 15 with the structure applied thereon is shown. The other components can be adopted identically from the previous embodiment. The planar antenna structure has two transmission and / or reception elements 33 arranged opposite one another, which are separated from one another by two further transmission and / or reception elements 35 arranged opposite one another. The circular surface of the substrate 15 is thereby divided into four quarter circles. By appropriately designing the connecting element, the four transmitting and / or receiving elements 33, 35 can be connected either individually or in pairs.

FIGS. 4 and 5 show two further exemplary embodiments of planar antenna structures, each of which has a large number of transmission and / or reception elements 37. It is also in these embodiments for the sake of simplicity, only the substrate 15 is shown with the structure applied thereon. The other components can be adopted identically from the previous embodiment.

Each transmitting and / or receiving element 37 occupies a small area compared to the substrate surface and is symbolically represented by a box. Of course, each transmitting and / or receiving element 37 can also be connected individually here, for example by A corresponding number of bores through which the connection is made are provided in the dielectric layer 23.

Preferably, however, several transmission and / or reception elements 37, e.g. according to their function. For example, As shown in FIG. 4, all transmitting and / or receiving elements 37 arranged in the right part of the substrate and all transmitting and / or receiving elements 37 arranged in the left part can be interconnected to form a functional block. The assignment is symbolically entered in FIG. 4 by marking one half of the transmitting and / or receiving elements 37 with a cross.

In the exemplary embodiment shown in FIG. 5, such an assignment to function blocks is also marked by crosses. Two function blocks are provided here and the associated transmitting and / or receiving elements 37 are arranged in a chessboard-like manner, nested within one another.

The embodiment of the antenna according to the invention with a planar antenna structure which has at least two transmission and / or reception elements offers a high degree of flexibility with regard to the use of the individual transmission and / or reception elements.

When measuring a fill level in a close range in front of the antenna 7, at least one of the transmitting and / or receiving elements preferably serves exclusively as a receiver. It is thereby achieved that an interference signal caused by the transmission has decayed at the receiver much more quickly and has a much lower amplitude than would be the case if this transmission and / or reception signal were also used for transmission. Accordingly, the block distance of the level measuring device designed in this way is smaller. The simplest form in which this can be implemented is shown in FIG. 6 in the form of a basic circuit diagram. All existing transmission and / or reception elements are divided into those which serve exclusively as transmitters designated as S in FIG. 6 and those which serve exclusively as receivers designated E in FIG. 6. The transmitters S are fed by the microwave generator 9 and the receivers E forward received signals received by you to the reception and evaluation circuit 41. The reception and evaluation circuit 41 determines the running time of the microwaves from the level measuring device to the product surface and back and determines the instantaneous level from this. This requires a time reference, which is only symbolically represented in FIG. 6 by a reference time t ₀ . The reference time t ₀ is, for example, a trigger pulse which at the same time triggers the transmission of a microwave pulse and starts an internal time measurement in the reception and evaluation circuit 41.

For example, in the exemplary embodiment shown in FIG. 2, the first transmitting and / or receiving element 11 serve exclusively as a transmitter and the second transmitting and / or receiving element 13 exclusively as a receiver. In the exemplary embodiment shown in FIG. 3, the two transmitting and / or receiving elements 33 can only serve as transmitters and the two transmitting and / or receiving elements 35 can only serve as receivers. In the exemplary embodiments illustrated in FIGS. 4 and 5, the transmitting and / or receiving elements 37 provided with a cross can only serve as transmitters and the transmitting and / or receiving elements 37 without a cross can only serve as receivers. The separation of transmitter and receiver results in a significant reduction in the block distance.

In the case of a measurement in the far range in front of the antenna 7, however, less transmission power is available in this case. While the separation between transmitter and receiver in the near range is very important, it is irrelevant for a measurement in the far range, since the running time of the microwaves for these measurements is long compared to the time in which an interference signal caused by the transmission has decayed. Therefore, when measuring a fill level in a remote area in front of the antenna 7, all the transmitting and / or receiving elements are preferably used as transmitters and receivers. A measure of whether a measurement result is in the near range or in the far range is, for example, the block distance that the fill level measuring device has when all transmit and / or Receiving elements serve as transmitters and receivers. A test measurement can therefore be carried out during commissioning, which serves to determine the measuring range in which the fill level is. Then, in normal measuring operation, the level last measured, for example, can be used to determine this. As soon as the fill level comes close to the close range, at least one transmitting and / or receiving element is used exclusively as a receiver. In the far range, all transmitting and / or receiving elements are used as transmitters and receivers.

FIG. 7 shows a basic circuit diagram of a fill level measuring device according to the invention, in which a part of the transmitting and / or receiving elements serve exclusively as a transmitter and the other transmitting and / or receiving elements serve exclusively as a receiver for measurements in the close range, and all transmitters are used for measurements in the far range - And / or receive elements serve as transmitters and receivers.

Here, the microwave generator 9 is connected via a directional coupler or circulator 43 to those transmitting and / or receiving elements denoted by S in FIG. 7, which serve exclusively as transmitters for measurements in the close range. Correspondingly, the reception and evaluation circuit 41 for measurements in the close range is connected to those transmit and / or receive elements designated E in FIG. 7, which serve exclusively as receivers for measurements in the close range.

For a measurement in the near range, the basic circuit diagram thus corresponds to the basic circuit diagram shown in FIG. 6. In the case of a measurement in the far range, on the other hand, all of the transmitting and / or receiving elements are interconnected via a line 45 connecting the two functional blocks. The line 45 is provided with an electronic switch 47, which is open for a measurement in the close range and closed for a measurement in the far range. Correspondingly, a switch 49 is provided in the connection between the receiving and evaluating circuit 41 and the transmitting and / or receiving elements E that serve exclusively as a receiver for a measurement in the near range, which switch 49 is closed for measurements in the near range and is open for measurements in the far range. For measurements in the far range, the signals received by the transmitting and / or receiving elements E are transmitted the line 45 is brought together with the signals received by the transmitting and / or receiving elements S and led via the directional coupler or ziculator 43 via a connection 53 provided with a switch 51 to the receiving and evaluating circuit 41. The switch 51 is open for a measurement in the near range and closed for a measurement in the far range.

The switches 47, 49, 51 are controlled by a control circuit 55, which is connected to the reception and evaluation circuit 41. The instantaneous fill level is determined in the reception and evaluation circuit 41 and e.g. compared to a reference value. If the fill level is above this reference value, the subsequent measurement is carried out as a measurement in the near range in that the control circuit 55 sets the associated switch positions by means of control signals. If the fill level is below this reference value, the subsequent measurement is carried out as a measurement in the far range in that the control circuit 55 sets the associated switch positions by means of control signals. In order to prevent a constant change of measuring mode in the border area at the transition from the close-up area to the far area, a hysteresis function can be installed here, e.g. two different reference values are used, with a drop below the lower reference value only causing a change from the short-range mode to the long-range mode and only a transition from the long-range mode into the short-range mode causing the upper reference value to be exceeded.

8 shows a further basic circuit diagram for a level measuring device according to the invention. In this fill level measuring device, at least one transmitting and / or receiving element S is used exclusively as a transmitter. These transmitting and / or receiving elements S are connected to the microwave generator 9. The remaining transmission and / or reception elements E serve exclusively as receivers and are connected to an input of the reception and evaluation circuit 41. In addition, a difference-forming unit 57 is provided, which determines a difference signal which corresponds to the difference between transmission signals applied to the transmitters and reception signals received by the receivers. For this purpose, an input of the difference-forming unit 57 is connected to the microwave generator 9 and a further input is connected to the transmitting and / or receiving elements E. The difference can be made either by Corresponding direct superimposition of the signals take place or by the incoming signals being recorded and recorded in digital form. The difference between the recorded data is then carried out, for example, using a microcomputer. Preferably, a delay unit 59 and a damping unit 61 are provided, which allow the signal coming from the microwave generator 9 to be delayed and damped by an adjustable delay time T and a damping factor ά. These rescalings can either be carried out in an analog manner by means of corresponding electronic components or can be carried out arithmetically on the stored data.

The difference signal serves to improve the measurement accuracy and is fed to the reception and evaluation unit 41 for this purpose. For example, a small crosstalk existing despite the separation of transmitters and receivers from the transmitters to the receivers can be almost completely corrected by correspondingly superimposing the received signals with the difference signals. The required data of the level measuring device, e.g. The delay time T and the damping factor ά can be determined by means of a factory calibration and stored permanently in the device.

Claims

claims

1. Microwave level measuring device for measuring a level of a filling material (5) in a container (1) with

- A microwave generator (39), and

- an antenna (7) with a planar antenna structure,

- Which serves to send the microwaves in the direction of the filling material (5) and to receive microwaves reflected on a filling material surface,

- In which the planar antenna structure has at least two transmitting and / or receiving elements (11, 13, 33, 35, 37, S, E).

2. Microwave level measuring device according to claim 1, in which the transmitting and / or receiving elements (11, 13, 33, 35, 37) are each located in a partial area of the antenna (7).

3. Microwave level measuring device according to claim 1, wherein the transmitting and / or receiving elements (37) are arranged nested in one another.

4. Microwave level meter according to claim 1, wherein at least one of the transmitting and / or receiving elements (E) is used exclusively as a receiver for measuring a level in a close range in front of the antenna.

5. Microwave level measuring device according to claim 1 or 4, in which all the transmitting and / or receiving elements (S, E) serve as transmitters and receivers for measuring a level in a remote area in front of the antenna.

6. Microwave level measuring device according to claim 1, in which at least one transmitting and / or receiving element (S) serves exclusively as a transmitter and the other transmitting and / or receiving elements serve exclusively as a receiver (E) and in which a difference signal is determined , which corresponds to the difference between transmission signals applied to the transmitters and reception signals received by the receivers.

7. Microwave level meter according to claim 1, wherein the microwaves to be transmitted have frequencies greater than 20

GHz.