RU2579634C2 - Radar waveguide level meter with waveguide pair - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention proposes two versions of a radar waveguide level meter, which are intended to measure level in plants, for example in vessels, excess-pressure boilers and silos. This level meter includes a receiving-and-transmitting unit, a waveguide pair and communication elements. The receiving-and-transmitting unit is located outside the plant and includes a processing unit, a modulator, a transmitter and a receiver. The waveguide pair is connected through the communication elements to the receiving-and-transmitting unit. The waveguide pair is attached apart from microwave sealed lead-ins and excited with vibrators connected to the receiving-and-transmitting unit via coupled lines. The coupled lines pass from the transmitter and the receiver through the microwave sealed lead-ins and end with vibrators that excite a waveguide line and pick up a level signal from it. A load on the end of the waveguide line can be either a reflector or an absorber.

EFFECT: invention allows simplifying the design of microwave sealed lead-ins and protecting them against loads acting on the waveguide pair, reducing level of losses and interference from structural elements; addition of a capacitance stage improves operation of the device in complex environments.

4 cl, 6 dwg

Description

The technical field to which the invention relates.

The present invention relates to radar (radar) level gauges with probes (level gauge GWR - Guided Wave Radar), designed to measure the current level in various installations: tanks, tanks, silos, including those with high pressures and temperatures.

The inventive device mainly uses the method of emission / reception of ultra-wideband signals with linear frequency modulation (FM / LFM, analogue of FM / CW).

State of the art

Guided Wave Radar Level Gauges (1) based on single-wire lines or transmission lines (Goubau line, single line probe, transmission line), hereinafter referred to as waveguide lines (VL or OVL), 2) on the basis of two-wire lines, are known then a waveguide pair (VP), from symmetrical two-wire (twin line, coupled transmission line) to asymmetric coaxial lines.

The choice of single-wire (VL) or two-wire (VP) lines is determined by the medium being measured - the OHL is structurally simpler, the VP dissipate less energy, have greater sensitivity (for media with low e) and, accordingly, an increased range of action. The energy of the field of the waveguide pair is concentrated between two VP conductors and around them, for example, as shown in the article by I. Goncharenko (1). The property of a waveguide pair to vary wave impedance to a wide extent is used in US 7636059 (2). Despite the noted advantages of two-wire lines, their use in microwave radar level gauges is limited.

In the application US 2012/0319891 A1 (3), waveguide pairs are proposed to be connected to the transceiver via an RF transformer or through a microwave ring on a printed circuit with quarter-wave isolation of the shoulders. But there are no examples of the practical use of RF transformers in microwave radar level gauges, just like microwave rings, because of their mismatch in a wide frequency band of the spectrum of real signals. In addition, the application does not have a solution for decoupling the transmitter and receiver when connecting them to a waveguide pair.

In the patent US 7827862 (4), as in a number of other known sources, the waveguide pair is formed, for example, by an active conductor connected to a transceiver and a passive conductor (conductors) connected to the housing (flange) of the device. Such a solution does not allow to achieve sufficient matching of the line with the source (receiver) of the signal due to its asymmetry, since the connection points of the active and passive conductors of the line are spaced in space and in area. A common drawback of all known radar level gauges is the complexity of the design of sealed microwave junctions (in Russian sources - microwave pressure seals), which make up a single design with overhead lines. The microwave pressure seal must have acceptable coordination in the frequency domain and at the same time must be mechanically strong, designed for high pressure and temperature, and the load of the medium on the overhead line can reach several tons. An example of the complexity of the transition design is US 7467548 (7).

Indeed, for all radar level gauges, a transmitter (signal generator) and a receiver are connected to the upper point of the active conductor of the airspace, therefore it is isolated from strong structural elements. The tension force of the VP wires, created by the material of the medium, and from the influence of excessive pressure, lies on the insulators of the microwave pressure glands. In practice, in order to maintain the strength of the pressure glands, it is necessary to violate the radio frequency matching, for example, by the cylindrical insert 27 in the patent (7). There are actually several points of frequency mismatch inside the structure in the patent (7), not counting the external mismatch at the input and output of the microwave pressure glands.

Analysis shows that RF mismatch is present in all known devices. The "pay" for this is a significant complication of signal processing. None of the known radar waveguide level gauges can work "blindly" in contrast to the functional counterparts - airborne radio altimeters. As a result, any manufacturer is forced to embed a graphical display of echo signals, enter complex menus and algorithms for setting devices on specific objects, declaring this as a product advantage. The benefit of the built-in graphical display, of course, is, for example, during routine maintenance, but this task can be solved remotely.

A disadvantage of the known radar level gauges is the "virtuality" of zero range - it is determined, as a rule, by the top of the pulse reflected from the lower end of the microwave hermetic junctions. This means that the transition must always be inconsistent in order to fix the level reference.

Almost all produced radar level gauges with airspace use the TDR - Time Domain Reflectometry pulse method, emitting short pulses with a duration of 0.5 - 1 ns. There are many sources describing the advantages of this method, as well as other methods, for example, the author Peter Devine, the website of Vega (5), and the newsletter of Magnetrol (6). However, the radar method with continuous emission of signals with frequency modulation (FM radar, or in another way the FM method) is not inferior to the TDR method, and in some applications it can be more effective, surpassing the TDR method in accuracy and reliability of measurement. This was made possible thanks to the development of microwave digital systems and devices, for example, the release of VCO chips and frequency synthesizers such as ADF4150HV and others.

Among the known analogues of the prototype of the claimed invention (the design of waveguide lines and other nodes) is not found.

Disclosure of invention

The problem solved by the claimed invention is: 1) to simplify the design and improve the coordination of microwave pressure glands by removing the load from them, 2) to increase the strength of the waveguide pairs by fixing the upper ends of the waveguide pair on a solid base and offset the point of communication (power) of the waveguide pair with source (receiver) of the signal, 3) in the exact definition of the physical zero point of the range for reading the level, 4) in obtaining new capabilities of the level gauges by dividing the waveguide pair into the emitting and receiving lines and added a capacitive channel.

The invention consists in a method for attaching and exciting a waveguide pair and applying the FM method. The upper ends of the VP can be attached to any conductive base by welding and in another way, at the lower end of the VP, a load can be attached, which can also act as a reflector or absorber.

A waveguide pair in the microwave range is excited at a distance of a few centimeters from the conductive base using elements (conductors), conventionally called vibrators. The signals supplied from the transmitter (microwave generator) to the VP and reflected from the boundaries of the medium, arriving at the receiver (microwave detector), are transmitted via short connected lines that can be integrated into microwave pressure seals.

The zero position of the range of the level gauges or the reference point when setting the level are determined. This zero point, subject to coordination of the connection (excitation) of the waveguide pair, is its attachment point.

The separation of the waveguide pair into the emitting and receiving lines reduces, for example, by an order of magnitude or more the level of structural noise created by the fastening elements, as well as the influence of sticking of the material of the medium on the conductors.

The introduction of a capacitive sensor into the device expands the possibilities of registering complex media, for example, determining the interface between media increases the reliability of the level measurement assessment.

Brief Description of the Drawings

Figure 1 - Radar waveguide level gauge. Mounting a waveguide pair on a conductive pad inside the unit. Universal option, including installations with high pressure and high temperature.

Figure 2 - Radar waveguide level gauge. External mounting of a waveguide pair on a conductive pad. The preferred option for installations with high temperatures at low pressures.

Figure 3 - Radar waveguide level gauge. External mounting of a waveguide pair on an insulator. The preferred option for high loads of the medium on the waveguide pair, at low temperatures and low pressures.

Figure 4 - Radar waveguide level gauge. Option with separate transmitting and receiving lines.

Figure 5 - Radar waveguide level gauge with a capacitive channel.

Figure 6 - 3D-model of the design of the fastening and power of the waveguide pair.

The implementation of the invention

Given the wide variety of installations with different processes (operating modes), radar waveguide level meters (RVU) with a waveguide pair can have different designs. The waveguide pair for some versions is attached and, accordingly, excited inside the installation, for others - outside. In figures 1-4 presents the most preferred versions of the HLD, we denote them hereinafter as the first, second, third and fourth options.

In the first (universal) version of the RGF in figure 1, the upper ends of the lines of the waveguide pair 12, 13 are mounted directly inside the installation to the base of the flange or the conductive surface (cover of the installation) 14. At the lower end of the waveguide pair, a load 15 is attached, which can be a reflector or absorber. The transmitter-receiver unit 1, located outside the installation, contains a processing unit 2, a modulator 3, a transmitter 4, and a receiver 5. The output of the transmitter and the input of the receiver are connected to the vibrator pairs 8 and 9 through the microwave pressure inputs 10 and 11, which are connected located at the excitation points of lines 12, 13 of the waveguide pair.

In the second embodiment, in figure 2, the upper ends of the lines of the waveguide pair 12, 13 are mounted on a metal support 17 outside the installation. The output of the transmitter and the input of the receiver through connected lines 6 and 7 are connected to the vibrating pairs 8 and 9, also located outside. The conductors 12, 13 of the waveguide pair pass into the installation through the pressure glands 10 and 11.

In the third embodiment, in figure 3, the upper ends of the lines of the waveguide pair 12, 13 are mounted on a dielectric support 18 outside the installation. The vibrator 8 is connected directly to the output of the transmitter 4, the vibrator 9 is connected to the input of the receiver 5.

The fourth option in figure 4 differs from the first in the arrangement of the vibrator pairs 8 and 9 for separate excitation of the lines of the waveguide pair 12, 13.

In figure 5, a capacitive sensor 19 connected to the processing unit 2 is introduced into the transceiver unit 1. The support platform 17 is divided into two parts: 17a and 17b, the input of the capacitive cascade 19 is connected to the support platform 17a.

Declared RVU work as well-known radar aircraft altimeters and FM level meters and similar products using the method of emission and reception of continuous signals.

Common to all device options is the following:

1) Rigid fastening of the beginning of the waveguide pair on the supports 17, 18 or on the base 14 allows you to remove the load created by the medium on the VP 12-13, which can reach several tons, from the microwave pressure glands 10 and 11. Accordingly, the design is simplified and the coordination of microwave pressure seals and the device as a whole is improved.

2) At the locations of the vibrators (vibrator pairs) 8 and 9, a mixture of the reference signal and the signal reflected from the boundary of the layer 16 or from the end of the line with the load 15 is formed, which can serve as a reflector for radiolucent media. The reference signal is part of the transmitting signal leaking from the transmitting vibrator 8 (vibrator pair) to the receiving vibrator 9 (vibrator pair). The mixed signal enters the receiver 5, where it is converted into a difference signal, from which, after filtering, a signal with a frequency corresponding to the measured range is extracted in the processing unit 2. Processing unit 2 synchronizes the scan cycle of modulator 3, which, using feedback from the PLL, forms a linear frequency tuning of the transmitter 4.

3) Vibrators are conductors that are connected to the VP for its excitation. Vibrators 8, 9 at the point of supply (excitation) of the VP can overlap with a slight overlap of the conductors of the VP. A thin layer of insulation between the vibrators and the conductors of the VP allows you to cut off the low-frequency and high-frequency noise induced in the line from microwave signals. Vibrators can be in the form of triangular plates, which provide ultra-wideband (UWB) matching of coupled lines with the VP. The location of the vibrators 8 and 9 is located at the point of excitation (power) of the waveguide pair 12-13 and is selected according to the maximum matching VP. It was experimentally determined that in the frequency band from 1 GHz to 10 GHz, the excitation point is a few centimeters from the upper end of the waveguide pair.

4) As the connected lines 6 and 7, well-known differential pairs are used - two-wire lines with a signal bandwidth up to terahertz and adjustable wave impedance. The field of the coupled line is concentrated mainly between the conductors of the line and decreases sharply behind them, which makes it possible to make compact (small-sized) microwave pressure glands 10, 11 based on the connected line, designed for high pressures and temperatures.

5) The load at the end of the VP intended for its tension can also act as a reflector or absorber, the choice of which depends on the reflectivity and radio transparency of the measured media.

6) It was experimentally verified that the VP attachment point — the base 14, the support platform 17 and 18 — is the zero point of the range (distance) from which the level measured to the boundary of the medium layer 16 is measured.

In the first embodiment of the device in figure 1, the waveguide pair 12-13 is rigidly fixed inside the installation on the base 14, which may be a flange, a cover, the housing of the installation, etc. The distance between the conductors of the VP is uncritical and is selected taking into account specific design solutions, for example, from 3 to 10 cm. At the points of excitation of the VP, 1-5 cm away from the base 14, there are vibrating pairs 8 and 9, exciting (supplying and removing the signal) VP To reduce reflections in the connected lines 6 and 7, low-resistance chip resistors can be installed at their ends from the side of the transceiver unit 1 and in front of the vibrators.

For installations with a slight overpressure, preferred devices are devices in which the VP is attached and driven externally. The device in figure 2 is preferable for installations with high temperatures of the measured media.

The device in figure 3 with the excitation of the VP by single vibrators 8 and 9 is preferable for installations with a high mechanical load on the VP, for example for installations with bulk materials. Due to the minimum of microwave elements in the VP excitation circuit, it has the lowest level of intrinsic noise and high sensitivity to weak reflections.

The figure 4 shows an example of a device with the separation of the conductors of the waveguide pair into a radiating (transmitting) line 12 and a receiving line 13. The vibrators 8a and 9a are located at the excitation points of the lines 12 and 13, and the vibrators 8b and 9b are in free space. The quality of matching of the connected lines is improved if low-resistance chip resistors are turned on between the ends of the connected lines 6 and 7 and the vibrators 8b and 9b. It would be possible to increase the radiation and reception potential by connecting balancing transformers (baluns) at the ends of connected lines 6 and 7, but this is not justified first of all because the device itself has a large margin (redundancy) of potential.

Separation of the lines into radiation and reception dramatically reduces the level of structural interference arising, for example, from a mismatch at the points of attachment of the microwave pressure glands, as well as from the mounting posts that hold the lines laid with bends along the side of the ship. Constructive interference is practically not transmitted from one line to another. Variants of devices VP with separate radiation and reception may be different.

Figure 5 shows an example of a device with a capacitive sensor channel 19. As a sensor 19, you can use a simple chip comparator with positive feedback (hysteresis) at a non-inverse input. The inverted input of the comparator is a capacitive circuit and is connected to the waveguide line 12. The waveguide line 13 can serve as a grounded electrode of the capacitive circuit 12 in installations with non-metallic walls. The waveguide line 12 with the support pad 17a is isolated from the base 14, the support pad 17b does not require isolation. In conductive media, the conductor of the waveguide line 12 must have an insulating coating (sheath).

The use of a capacitive sensor 19 not only increases the reliability of level measurements on the main radar channel, but also allows you to detect delamination and changes in the properties of the medium 16, for example, the transition of water to superheated steam in installations with high pressure and temperature.

The figure 6 presents a graphical 3D model of the waveguide pair 12-13 with communication lines 6 and 7 passing through the pressure glands 10 and 11, and vibrating pairs 8 and 9 mounted on the flange 14.

Bibliography

1. Goncharenko IV, article: http://dl2kq.de/ant/3-41.htm

2. US patent US 7636059 B1.

3. US patent US 2012/0319891 A1.

4. US patent US 7827862 B2.

5. Stillstandmessung mit Radar Leitfaden für die Prozessindustrie, Peter Devine http: //www.vega.corn/downloads/VEGA-Radarbuch.pdf

6. Magnetrol bulletin: http://www.levelswitches.com/pdfs/l/41-229.pdf

7. US patent US 7467548 B2.

Claims (4)

1. A radar waveguide level gauge containing a transceiver, including a processing unit, a modulator, a transmitter and a receiver, a waveguide pair, characterized in that it contains two connected lines, vibrators and two microwave pressure glands, the upper ends of the waveguide pair mounted on a metal base inside the installation, the vibrators are located at the points of excitation of the waveguide pair and are connected by connected lines passing through the microwave pressure glands, with the output of the transmitter and the input of the receiver, the load at the end of the waveguide pair t be a reflector or absorber. 2. A radar waveguide level gauge containing a transceiver, including a processing unit, a modulator, a transmitter and a receiver, a waveguide pair, characterized in that it contains vibrators and two microwave pressure glands, the upper ends of the waveguide pair mounted on a metal or dielectric support outside the installation, waveguide conductors the pairs pass through the microwave pressure glands inside the unit, the vibrators are located at the points of excitation of the waveguide pair outside the unit and are connected to the output of the transmitter and the input of the receiver and, the load on the end of the waveguide pair can be a reflector or absorber. 3. The radar waveguide level gauge according to claim 2, characterized in that one conductor of the waveguide pair is the emitting line, and the second conductor is the receiving line. 4. The radar waveguide level gauge according to claim 2, characterized in that it contains a capacitive sensor, the input of which is connected to the waveguide line, and the output to the processing unit.

Images