WO2001096900A1

WO2001096900A1 - Method and device for improving the temperature stability and resistance to ageing of radar level meters using a mechanical reference

Info

Publication number: WO2001096900A1
Application number: PCT/DE2001/002133
Authority: WO
Inventors: Rolf Gluth
Original assignee: Eads Deutschland Gmbh
Priority date: 2000-06-10
Filing date: 2001-06-07
Publication date: 2001-12-20
Also published as: DE10028864A1; AU2001268946A1

Abstract

A signal part of a wave that is emitted by a radar level meter, which is reflected on a mechanical reference is evaluated in order to improve the temperature stability and resistance to ageing of said radar level meter. This reflected signal part is evaluated in the evaluating electronics unit in the same way as the radar echo. In addition to the electrical distance del filling to the filling surface that is determined, the electrical distance del reference to the mechanical reference is determined so that the level of the filling hf can be determined from the known distances between the radar system and the container floor hM and between the radar system and the mechanical reference dreference. To this end, a mechanical reference is introduced into the transmit-receive branch of the radar level meter, so that a significant signal part of a wave is reflected on the same. The mechanical reference is preferably a dielectric object which is integrated into the antenna feed of the radar level meter.

Description

Method and device for improving the temperature stability and aging resistance of radar level meters by means of a mechanical

reference

The invention relates to a method and a device for improving the temperature stability and aging resistance of radar level gauges, according to the features of claim 1.

A large number of measuring methods and devices using radar systems have been proposed for measuring the fill level of containers in recent years. The documents DE 3830992 and EP 0887658 describe devices and methods for level measurement using FMCW radars. Here, the distance between the radar system and the surface of a filling material is derived from the frequency difference between the current frequency of the transmitting oscillator and the frequency of the received wave, which is proportional to the running time of a transmitted wave.

DE 44 19462 C2 describes a radar level meter in which the radar signal is conducted into a still pipe to increase the measuring accuracy. Through the stillpipe, e.g. dampened the movements of the filling material. The signals reflected by the reference elements arranged in the surge pipe are used to calibrate the level and to check the function of the radar level meter.

In the level meter described in DE 28 12 871, a beat signal is determined from the signal component reflected by the filling material and the signal component reflected from reference elements, the frequency of which is determined. The level is then determined from the frequency of the beat signal. A level meter for reducing interference signals is described in DE 42 41 910 A1. Here, the measurement signal received by a receiving antenna is compared with a threshold value. This allows interference signals to be filtered out of the measurement signal, for example due to contamination of the antenna.

The temperature effect on the active and passive components of an FMCW radar system creates temperature-dependent distance sensors in particular when using devices that work at high frequencies. These measurement errors are independent of the absolute distance between the radar system and the product and are expressed in a distance offset. This offset is of the order of a few millimeters. Such a measurement error is particularly not permitted for devices for level measurement which are to be used as verifiable devices according to the OIML Recommendation R85.

In US 5406842 Locke describes a device within a radar system which essentially consists of a delay line of known length. The method for calibrating the level meter described in the document compares the distance traveled by a wave sent into the empty measuring space with the transit time of this wave through the delay line. The integration of such a delay line in addition to the still necessary incorporation of a comparison facial expression for the calibration of the system represents a not inconsiderable implementation effort.

It is therefore an object to find a method and a device which make it possible to compensate for the effects of temperature fluctuations and signs of aging on the active and passive components of a radar system used for level measurement.

The object is achieved in that, from knowledge of the distance of the radar system from the mechanical reference d _RΘfe.enz, changes in the electrical distance d _e ι_ _R efe.e _n z can be recognized and taken into account in the calculation of the filling level of the filling material h _f . In an advantageous manner, in addition to the determined electrical distance d _Θ ι_ _Fü iiung to the filler surface, the electrical distance d _e ι_Refe.enz for mechanical reference is determined so that from knowledge of the distances between the radar system and container bottom h _M and between the radar system and mechanical reference d _{r Θ} rence the filling level of the filling material hf can be determined.

An advantageous device is characterized in that a mechanical reference is introduced into the transmit / receive branch of the radar full-level sensor, so that a significant signal component of a wave is reflected thereon. It is advantageous to integrate a dielectric object into the antenna feed of the radar level sensor as a mechanical reference. The object is, for example, a disc or a cylinder and preferably has a bead which is inserted into a groove in the waveguide. It is advantageous here to design the bead in a ring shape.

The invention is explained in more detail below on the basis of exemplary embodiments and figures. It shows

FIG. 1 shows a schematic illustration of an arrangement of a radar level meter suitable for carrying out the method according to the invention, as is known from the prior art, and

Figure 2 is a schematic representation of an embodiment of an advantageous device in which the reference target is integrated in the antenna feed. Figure 3 shows a cross section through a mechanical reference in the form of a disc.

FIG. 4 shows a cross section through an antenna arrangement in the antenna feed of which a mechanical reference in the form of a cylinder tapering at its ends is integrated. FIG. 5 shows a cross section through a further advantageous arrangement of a cylindrical reference (as known from FIG. 4) within the antenna arrangement. The schematic representation of an arrangement of a radar zero level meter, as is suitable for carrying out the method according to the invention, is shown in FIG. The arrangement is based on an arrangement known from the prior art of a frequency-modulating radar system (FMCW or

SFMCW-RADAR) and a container (tank). A high-frequency signal generated by the generator (101) is passed via a line (102) to a directional coupler (104). From there it reaches the surface of a filling material (108) via an antenna feed (105) and the antenna (106) as a radar beam (107) through the free space. The antenna feed (105) usually consists of several individual components. To simplify matters, it is shown here as a single component. The radar echo (110) resulting from the reflection of the radar beam on the surface (108) is conducted via the antenna (106), the derivative (105), the directional coupler (104) and the feed line (111) to the mixer (112). There it is compared (multiplied) with a reference signal supplied via the line (103) and in a fixed relationship to the high-frequency signal generated in the generator (101). As a result, a signal (204) is converted in the mixer (112) with a frequency proportional to the running time of the emitted radar wave. This signal (204) is fed to the evaluation electronics (114), in which the distance calculation takes place.

The thus determined "electrical" distance is d _e ι_ _feet iiung. In order to calculate the quantity d _rad ar required for calculating the filling level h _f , the quantity dj_ei determined by the transit time of the radar signal and the radar echo in the device itself must be known. The equations apply: d adar = d _e | _Fülluπg "dj_ _e | GI.1

hf =

- deMFülllung ⁺ dj_ _e | GI.2

Assuming that dj_ _e ι is a constant, it is sufficient to determine this value once for a series or, depending on the sample scatter of the components used, once for each radar device in a series by calibration. However, the assumption that dj_ _e ι is a constant is only approximate. In the case of very precisely working devices with a resolution of the order of one millimeter, deviations due to temperature influences are particularly evident also due to aging of components. Advantageously, a radar target is therefore to be attached to a defined distance from the radar system and the echo reflected by this target is to be evaluated.

Exemplary devices suitable for carrying out the method according to the invention are described in the published documents DE 4327333 A1 and WO 9512113. The devices described therein describe the attachment of a microwave-permeable cutting disc (e.g. a dielectric plate of suitable thickness) between the antenna of the radar system and a liquid level. Such a cutting disc is always necessary anyway if the liquid in a container has a damaging effect on the antenna due to its vapor, in particular due to corrosion. The cutting disc thus separates the volume of the container in which the liquid is located from the volume receiving the antenna. If the microwave signal emitted by the antenna hits the cutting disc, part of the energy of the microwave signal is reflected back from the upper surface of the cutting disc to the radar system. Reflections on the underside of the cutting disc, which return to the radar system, superimpose the mentioned signal in such a way that the amplitude of the sum signal depends on the thickness and the dielectric constant of the cutting disc. The reflected signal can be optimized by suitable selection of these values.

The method according to the invention now makes use of the signal reflected on the cutting disc in such a way that it feeds this reference echo via an antenna, the antenna feed, the directional coupler and the connection to the mixer and compares it there with the reference frequency. The signal generated in this way is processed in the evaluation electronics in the same way as the radar echo. Since the reference and radar echoes follow the same paths, the measured values d _e ι_R _Θ reference and d _e ι_Füiiung determined in this way are subject to the same temperature-dependent fluctuations of dj ei- The following applies: dj el ⁼ d _e l_Referenz- d reference GI.3 Substituting into equation 2 gives hf = hwi -d _e | _filling ⁺ d _e ι_reference -dreference GI.4

This calculation rule is independent of di_ _e ι and is used in the solution according to the invention instead of equation 2 for calculating the fill level. Thus, temperature changes and aging influences on the way via dj_ _e ι no longer influence the measurement result.

Instead of using a cutting disc as a mechanical reference, it is nevertheless also possible to insert another object that reflects electromagnetic waves, for example a grating or a dielectric or metallic reflector, into the beam path between the antenna of the radar level meter and the surface of the filling material (108).

However, it is also conceivable in an inventive manner to implement a device containing a reference target in such a way that this reference target is integrated into the antenna feed near the antenna. Such a device, as also shown in detail by way of example in FIG. 3, can be produced in a mechanically simpler and more robust manner than that known from the prior art. The use of such a device for carrying out the method according to the invention is particularly advantageous because thermal and aging-related changes in the dimensions of the dimensions of the antenna (106) of the radar system are negligible with suitable dimensions.

Figure 2 shows schematically an embodiment of such a simple and robust variant of the device according to the invention. Here, the reference target (301) is integrated into the antenna feed near the antenna. Components that have remained the same as in FIG. 1 are provided with the same reference symbols. Instead of equation 3 the following applies: dj_ _e | = d _Θ | Reference ⁺ antenna el GI.5 Substituting in equation 2 provides

hp =

d _Θ l_Fülllung ⁺ d _e | _Referenz ⁺ dAntenne_el GI.6

This calculation rule only depends on the proportion d _An tenne_ei from dj_ _e ι. If the design is correct, the value d _an t _e nne_ei is subject to far fewer fluctuations than dj_ _e ι-

Figures 3 to 5 show possible advantageous installation positions and configurations of reference targets (301a) and (301b) as they can be integrated into the antenna feed.

FIG. 3 shows an advantageous embodiment of a reference target (301 b) suitable for an advantageous device, which has a ring-shaped retaining bead (310). The bead (310) serves as a mechanical holder and seal against the penetration of condensing vapors (here: by clamping between two components connected by means of a screw connection). The reflection takes place on the disk itself, the ratio of the thickness to the wavelength being important for the amplitude of the reflected signal. For this purpose, the bead (310) must be dimensioned in a suitable manner. The disc is provided with a concave trough (320), which points towards the antenna opening during installation and serves to drain condensate into the less critical edge areas of the antenna.

FIGS. 4 and 5 show further advantageous possible embodiments of an advantageous device. This is the configuration of the reference target (301 a) in the form of a dielectric cylinder which tapers at its ends and has an annular retaining bead (310). The holding bead (310) also serves mechanical and electrical purposes here. The reference target (301 a) has peaks. On the one hand for the sliding adaptation of the wave resistance to the antenna feed and the free space and on the other hand as a drain of condensate forming on the antenna into the tank.

At the installation positions of the reference target (301 a) shown in FIGS. 4 and 5, the disk-shaped reference target (301 b) can of course also be used Figure 3 can be installed. In an advantageous manner, however, it is also conceivable to introduce a reference target (301) in each of the two installation positions.

The design of the reference targets (301) is of course in no way restricted to the embodiments (301 a) or (301 b) shown in FIGS. 3 to 5.

The method according to the invention can be profitably expanded by using a plurality of reference targets (301) introduced into the transmit / receive branch. This not only allows an offset that may be present in the distance measurement to be corrected, but also by evaluating at least two transit times multiplicative error term (gain error) are eliminated. This is possible because the system knows the correct runtime difference with which an actual runtime difference can be compared.

Claims

claims

1. A method for improving the temperature stability and aging resistance of radar level meters, wherein the signal component reflected from a mechanical reference (201 or 301) of a wave emitted by the radar level meter is evaluated with regard to its transit time, characterized in that from knowledge of the distance of the radar system from the mechanical reference d _Re ference changes in the electrical distance d _e _f erence ι_Re detected and when calculating the fill level of the filling material hf can be taken into account.

2. The method according to claim 1, characterized in that the reflected signal component in the evaluation electronics is evaluated in the same way as the radar echo and the evaluation electronics, in addition to the determined electrical distance d _e ι_Füiiung to the filler surface (108) also see the electrical distance

the mechanical reference (201 or 301), so that, from knowledge of the distances between the radar system and the container bottom hj i and between the radar system and the mechanical reference d _R e _f erence the filling height of the filling h _f can be determined according to the relationship:

hf = hiυi - del_Füllung ⁺ d _e | _Referenz "dReferenz

3. The method according to any one of claims 1 to 2, characterized in that a multiplicative error term (gain error) in the distance measurement is eliminated by evaluating at least two signal propagation times which have arisen from reflections on two targets known with regard to their distance can be.

4. Device for carrying out a method according to one of the preceding claims, characterized in that at least one mechanical reference (201 or 301) is present in the transmit / receive branch of the radar level sensor, so that a significant signal component of a wave is reflected thereon ,

5. The device according to claim 4, characterized in that a dielectric object (301) is integrated as a mechanical reference in the antenna feed of the radar level sensor.

6. The device according to claim 5, characterized in that the mechanical reference has a retaining bead (310) which serves as a mechanical holder and for sealing against the ingress of condensing vapors.

7. Device according to one of claims 5 or 6, characterized in that the dielectric object (301 b) is a disc.

8. The device according to claim 7, characterized in that the thickness of the disc corresponds to an odd multiple of a quarter of the wavelength of the waves emitted by the radar level in the dielectric.

9. Device according to one of claims 7 or 8, characterized in that the disc has on one side a concave trough (320).

10. Device according to one of claims 5 or 6, characterized in that the dielectric object (301 a) is a cylinder.

11. The device according to claim 10, characterized in that the cylinder tapers at its ends.

12. The device according to one of claims 7 to 11, characterized in that the retaining bead (310) is designed such that it represents an electrical reflection point.