Radar level gauge and temperature-pressure compensation method

Abstract

This invention discloses a radar level gauge and its temperature and pressure compensation method, belonging to the field of level measurement technology. The method involves transmitting a radar signal into a container and receiving a mixed signal containing a reference echo from a reflector at a fixed distance and an echo from the liquid surface target; identifying the reference echo with predetermined characteristic signals and obtaining its characteristic frequency value; calculating the actual wave velocity in the current gaseous medium based on the characteristic frequency value and the known distance to the reflector; and finally using the actual wave velocity to calculate the actual liquid level height. This method can compensate for wave velocity errors caused by changes in the gaseous medium in real time and adaptively, eliminating the need for external sensors and improving the accuracy and reliability of level measurement.

Description

Technical Field

[0001] This invention relates to the field of radar detection technology, specifically to a radar level gauge and its temperature and pressure compensation method. Background Technology

[0002] Radar level gauges are widely used in industries such as petroleum, chemical, and power due to their advantages of non-contact measurement, high accuracy, and low maintenance. Their basic principle is: an antenna emits electromagnetic pulses, which are reflected off the surface of the measured medium and received by the antenna. The liquid level is calculated by measuring the travel time of the electromagnetic wave.

[0003] While the speed of electromagnetic waves in a vacuum is constant, in practical industrial applications, they need to propagate through gaseous media (such as air, steam, or gas mixtures) within a container. The propagation speed of electromagnetic waves in a medium is determined by the medium's relative permittivity and relative permeability. The permittivity of a gaseous medium is not a fixed value; it varies significantly with changing process conditions. Increased temperature and decreased gas density lead to a reduction in the number of polarized molecules per unit volume, resulting in a decrease in the permittivity. Conversely, increased pressure and increased gas density lead to an increase in the permittivity.

[0004] Existing radar level gauges are typically calibrated at the factory under standard operating conditions. However, when applied to high-temperature, high-pressure reactors, liquefied natural gas storage tanks, or power plant boiler drums, the wave velocity changes due to temperature and pressure variations. Calculating based on a fixed wave velocity would result in significant measurement errors, potentially even jeopardizing process control safety. Some instruments incorporate temperature and pressure compensation, but these are mostly indirect compensation based on empirical formulas. This involves collecting data from externally installed temperature and pressure transmitters and substituting it into theoretical formulas for correction. This open-loop compensation method relies on the accuracy of the temperature and pressure sensors and cannot capture the effects of real-time changes in the composition of the gaseous medium.

[0005] Another existing instrument uses two fixed reflectors with a known physical distance installed within the measurement blind zone. The actual wave velocity is directly calculated from the time difference of their echoes. The liquid level height is then calculated by further measuring the travel time of the electromagnetic wave. However, stray echoes from tank walls, supports, etc., can interfere with the reference echo, making it difficult to clearly identify the two reference echoes and reducing reliability. Summary of the Invention

[0006] The purpose of this application is to overcome the shortcomings of the prior art and provide a radar level gauge and a temperature and pressure compensation method.

[0007] The first aspect is the radar level gauge temperature and pressure compensation method, which includes the following steps:

[0008] S1. Transmit a continuous wave radar detection signal into the container and receive a mixed signal containing a reference echo from a fixed-distance reflector and a target echo from the liquid surface, wherein the fixed-distance reflector is located at a known fixed position inside the container;

[0009] S2. Perform mixing and feature extraction on the mixed signal to identify the reference echo with a predetermined spectrum or phase characteristics, and obtain the characteristic frequency value of the reference echo and the characteristic frequency value of the target echo.

[0010] S3. Based on the characteristic frequency value of the reference echo, the modulation parameters of the continuous wave radar detection signal, and the known physical distance of the fixed-distance reflector, calculate the actual propagation speed of the radar detection signal in the current gaseous medium of the container.

[0011] S4. Calculate the actual height of the liquid surface based on the actual propagation speed, the characteristic frequency value of the target echo, and the modulation parameters.

[0012] Furthermore, the predetermined spectrum or phase feature is an echo signal composed of multiple spectral peaks with a specific number of peaks and a specific frequency interval on the spectrum; the reference echo is identified by matching and comparing the extracted echo spectrum features with the parameters of the pre-stored predetermined features.

[0013] Furthermore, the fixed-distance reflector includes multiple reflective elements arranged in a non-equidistant manner along the radar beam direction, for generating multiple characteristic spectral peaks with the specific frequency spacing in the mixed signal spectrum.

[0014] Furthermore, step S3 specifically includes:

[0015] S31. Measure the actual frequency difference between adjacent characteristic spectral peaks from the identified multi-peak reference echo spectrum;

[0016] S32. Based on the known physical distance difference between each pair of adjacent reflecting units, the corresponding actual frequency difference, and the modulation parameters, multiple intermediate wave velocity values ​​are calculated respectively.

[0017] S33. The multiple intermediate wave velocity values ​​are verified and fused to obtain the actual propagation speed.

[0018] Furthermore, the verification and fusion processing in step S33 includes: calculating the consistency among the plurality of intermediate wave velocity values; if the consistency meets the preset conditions, then the average or weighted value of the plurality of intermediate wave velocity values ​​is taken as the actual propagation speed.

[0019] Furthermore, the formula for calculating the actual height of the liquid level is:

[0020]

[0021] H is the actual height from the radar antenna flange reference plane to the measured liquid surface. Let T be the characteristic frequency value of the target echo, T be the modulation period, and B be the frequency modulation bandwidth. This refers to the actual speed of transmission.

[0022] Secondly, a radar level gauge is provided for implementing the method of the first aspect, comprising:

[0023] The instrument body contains a signal transmitting and receiving unit and a signal processor;

[0024] An antenna, connected to the signal transmitting and receiving unit, is used to transmit radar waves into the container and receive echoes.

[0025] A fixed-distance reflector, fixedly installed at a known fixed position inside a container, is configured to generate a reference echo with a predetermined characteristic signal;

[0026] The signal processor is configured to perform the following operations:

[0027] Identify a reference echo with a predetermined characteristic signal from the fixed-distance reflector;

[0028] The actual propagation speed of radar waves in the current gaseous medium is calculated based on the reference echo.

[0029] The liquid level height is calculated and output using the actual propagation speed.

[0030] Beneficial effects: By using a reflector with specific frequency domain characteristics, the system can accurately lock onto the reference signal in complex tank echo environments, reducing noise interference; it can automatically and continuously correct wave velocity fluctuations caused by changes in temperature, pressure, and gas composition, eliminating systematic measurement errors caused by traditional methods that use fixed wave velocities or rely on inaccurate empirical formulas, improving the absolute accuracy of liquid level measurement, and is especially suitable for harsh working conditions with high temperature, high pressure, and variable media. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of the temperature and pressure compensation method of the present invention.

[0033] Figure 2 This is a schematic diagram of the installation structure of the radar level gauge of the present invention. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. The described embodiments are only some embodiments of the invention, not all embodiments. Based on the embodiments of the invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the invention.

[0035] To address the problems existing in the relevant prior art, this invention proposes a radar level gauge temperature and pressure compensation method. The principle and structure of this invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0036] Taking the liquid level measurement of a high-temperature and high-pressure steam-liquid storage tank as an example, the device configuration of the embodiments of this application is specifically described as follows: Figure 2 As shown, it includes radar 1 and reflector 2.

[0037] Instrument Selection: A 120GHz frequency modulated continuous wave (FMCW) radar level gauge is selected, with a measuring range of 0-25 meters, an operating temperature of 400℃, and an operating pressure of 15MPa. The transmitted signal is a linear frequency modulated continuous wave with a center frequency of 120GHz, a frequency modulation bandwidth of B=4GHz, and a modulation period (scan time) of T=20ms.

[0038] Reflector: An array of three small pyramidal reflectors (or high-reflectivity metal patches), whose physical distances from the antenna flange reference plane are respectively... , , ,and , , In the FMCW system, the echo from each reflector unit, after being down-converted by mixing, corresponds to an independent beat frequency peak in the baseband spectrum. Since the spacing is fixed and known, these three peaks form a spectral peak sequence with specific frequency intervals in the frequency domain, making them easily distinguishable from single or random spectral characteristics such as liquid surface or tank wall multipath. A rigid support is used to precisely fix the relative positions of multiple reflectors. The support is made of a low thermal expansion coefficient alloy (such as Invar alloy or 316L stainless steel) to ensure that the distance difference remains constant under harsh environments such as high temperature and vibration. The reflective surface uses a highly conductive metal to ensure a balanced signal-to-noise ratio for the echoes of each unit. Here, to avoid mutual obstruction between reflectors affecting the radar signal, their positions can be staggered by a certain amount.

[0039] Signal processing unit: Includes an orthogonal mixer, low-pass filter, high-speed ADC, and DSP / MCU main control chip. The ADC sampling rate must meet the Nyquist criterion and be several times the maximum expected beat frequency to accurately digitize the baseband beat frequency waveform; it has sufficient RAM / Flash memory to cache FFT spectrum data, pre-stored reflector spectrum feature templates, historical wave velocity records, and temperature and pressure compensation algorithm programs.

[0040] Please see Figure 1 The temperature and pressure compensation method of this invention:

[0041] During the factory calibration or on-site installation and commissioning of radar level gauges, the echo from the reflector is collected under calm, standard operating conditions (such as normal pressure and temperature air). After FFT transformation, its multi-peak characteristic waveform is extracted, and the center frequency and relative frequency interval ratio of each peak are extracted. This is then digitized and used as a reference template signal. It is stored in the signal processing unit. Alternatively, it can be stored in the signal processing unit in discrete Gaussian weighted form:

[0042]

[0043] Where K is the number of FFT points and N is the number of reflection units. This is the theoretical FFT frequency index corresponding to the i-th reflection unit under standard operating conditions. For amplitude-normalized spectral peak weights, The main lobe width parameter is used for windowing. Simultaneously, the theoretical frequency difference ratio sequence of adjacent spectral peaks is extracted and stored together. This template structure balances absolute frequency locking and relative ratio invariance, ensuring that even when temperature and pressure changes cause overall spectral shifts, the reference echo characteristics can still be accurately identified through frequency domain cross-correlation.

[0044] The radar level gauge transmits a linear frequency modulated continuous wave signal into the tank via an antenna. The signal propagates through the high-temperature, high-pressure gaseous medium inside the container, and is reflected successively by a fixed-distance reflector and the liquid surface, forming a mixed echo signal that is received by the antenna. The echo signal and the transmitted signal are orthogonally mixed inside the device and then low-pass filtered to obtain a baseband mixed signal containing both the reference beat frequency and the target beat frequency. .

[0045] Real-time acquired baseband mixed signal After preprocessing (such as adding a Hanning window, DC offset correction, and amplitude normalization), its spectrum is calculated. and compared with pre-stored multispectral peak feature templates Perform spectral cross-correlation matching calculation:

[0046]

[0047] in, This represents the actual physical frequency value corresponding to the k-th frequency point in the discrete FFT spectrum. For real-time mixing of echo signals at frequency Spectral amplitude at that location; To store the reference template signal at frequency The value at; It is a cross-correlation function. This is the frequency shift of the spectrum. When the spectrum Appearance in China When the spectral peak structures are highly similar, In the corresponding frequency shift A sharp peak is generated at this point. When this peak exceeds a preset matching threshold, it is determined that the reference echo of the fixed-distance reflector has been successfully locked.

[0048] By reading the frequency shift corresponding to the peak value and combining it with the FFT frequency resolution, the first characteristic frequency value of the first spectral peak in the reference echo is obtained. By using peak interpolation algorithms (such as parabolic fitting or FFT phase difference method), the actual frequency difference between adjacent peaks in the three spectra can be accurately measured. and Using a conventional CFAR peak detection algorithm, the strongest peak frequency of the liquid surface echo spectrum, i.e., the second characteristic frequency value, is identified and extracted from the echo spectrum. .

[0049] Intermediate wave velocity calculation: based on the FMCW distance-frequency linear mapping relationship Let T be the modulation period and B be the frequency modulation bandwidth. Substituting the known distance difference and the measured frequency difference, calculate the two sets of intermediate wave velocity values:

[0050]

[0051] Further calculation of the relative consistency error between the two intermediate wave velocity values :

[0052]

[0053] Will Consistency tolerance with preset (For example, set it to 0.001) Compare:

[0054] like If the reference signal is considered valid and not subject to strong multipath / bubble interference, the actual propagation speed is calculated: ;like If the reference signal is suspected to be affected by transient interference, the system automatically activates a fault-tolerant strategy: using the previous valid measurement cycle's... Values ​​are smoothly replaced or compared. and For the corresponding spectral peaks, select the calculated value with the higher signal-to-noise ratio as the current peak. Using real-time calibrated wave velocity Dynamic compensation is applied to the liquid surface beat frequency to calculate the final liquid level height.

[0055]

[0056] H is the actual height from the radar antenna flange reference plane to the measured liquid surface. This is the second characteristic frequency value (liquid surface beat frequency).

[0057] This embodiment utilizes a fixed reflector at a known location inside the container as a built-in scale. The reflector's echo directly and in real-time measures the actual propagation speed of electromagnetic waves in the current gaseous medium, and this speed is used to calculate the liquid level. This process forms an autonomous closed loop, automatically and continuously correcting for wave speed fluctuations caused by changes in temperature, pressure, and gas composition. It eliminates the systematic measurement errors inherent in traditional methods that rely on fixed wave speeds or inaccurate empirical formulas, improving the absolute accuracy of liquid level measurement. This is particularly suitable for harsh operating conditions involving high temperature, high pressure, and variable media.

[0058] It eliminates the need for external temperature and pressure sensors and their complex installation, wiring, and data fusion systems. This not only reduces system cost, complexity, and maintenance workload, but more importantly, it avoids secondary errors and single-point failure risks introduced by inaccurate single-point measurements from external sensors, unrepresentative installation locations, signal transmission delays, or malfunctions.

[0059] By employing reflectors with specific time-domain characteristics, the system can accurately lock onto the reference signal in complex tank-based echo environments. Simultaneously, by utilizing multiple reflection units to calculate multiple intermediate wave velocity values ​​and perform consistency checks, an inherent self-diagnostic function is achieved.

[0060] Based on the same inventive concept, this application also provides a radar level gauge, such as... Figure 2 As shown, it includes:

[0061] The instrument body 1 includes a signal transmitting and receiving unit and a signal processor; an antenna connected to the signal transmitting and receiving unit, used to transmit radar waves into the container and receive echoes; a fixed-range reflector 2, fixedly installed at a known fixed position inside the container, configured to generate a reference echo with a predetermined characteristic signal; wherein the signal processor is configured to perform the following operations:

[0062] Identify a reference echo with a predetermined characteristic signal from the fixed-distance reflector;

[0063] The actual propagation speed of radar waves in the current gaseous medium is calculated based on the reference echo.

[0064] The liquid level height is calculated and output using the actual propagation speed.

[0065] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for temperature and pressure compensation in a radar level gauge, characterized in that, Includes the following steps: S1. Transmit a continuous wave radar detection signal into the container and receive a mixed signal containing a reference echo from a fixed-distance reflector and a target echo from the liquid surface, wherein the fixed-distance reflector is located at a known fixed position inside the container; S2. Perform mixing and feature extraction on the mixed signal to identify the reference echo with predetermined spectrum or phase characteristics, and obtain the characteristic frequency value of the reference echo and the characteristic frequency value of the target echo. S3. Based on the characteristic frequency value of the reference echo, the modulation parameters of the continuous wave radar detection signal, and the known physical distance of the fixed-distance reflector, calculate the actual propagation speed of the radar detection signal in the current gaseous medium of the container. S4. Calculate the actual height of the liquid surface based on the actual propagation speed, the characteristic frequency value of the target echo, and the modulation parameters.

2. The method according to claim 1, characterized in that, In step S2, the predetermined spectrum or phase feature is an echo signal composed of multiple spectral peaks with a specific number of peaks and a specific frequency interval on the spectrum; the reference echo is identified by matching and comparing the extracted echo spectrum features with the parameters of the pre-stored predetermined features.

3. The method according to claim 2, characterized in that, The fixed-distance reflector includes multiple reflective elements arranged non-equidistantly along the radar beam direction, used to generate multiple characteristic spectral peaks with the specific frequency spacing in the mixed signal spectrum.

4. The method according to claim 3, characterized in that, Step S3 specifically includes: S31. Measure the actual frequency difference between adjacent characteristic spectral peaks from the identified multi-peak reference echo spectrum; S32. Based on the known physical distance difference between each pair of adjacent reflecting units, the corresponding actual frequency difference, and the modulation parameters, multiple intermediate wave velocity values ​​are calculated respectively. S33. The multiple intermediate wave velocity values ​​are verified and fused to obtain the actual propagation speed.

5. The method according to claim 4, characterized in that, The verification and fusion process in step S33 includes: calculating the consistency among the multiple intermediate wave velocity values; if the consistency meets the preset conditions, the average or weighted value of the multiple intermediate wave velocity values ​​is taken as the actual propagation speed.

6. The method according to claim 1, characterized in that, In step S4, the formula for calculating the actual height of the liquid level is: H is the actual height from the radar antenna flange reference plane to the measured liquid surface. Let T be the characteristic frequency value of the target echo, T be the modulation period, and B be the frequency modulation bandwidth. This refers to the actual speed of transmission.

7. A radar level gauge, characterized in that, For implementing the method as described in any one of claims 1-6, it comprises: The instrument body contains a signal transmitting and receiving unit and a signal processor; An antenna, connected to the signal transmitting and receiving unit, is used to transmit radar waves into the container and receive echoes. A fixed-distance reflector, fixedly installed at a known fixed position inside a container, is configured to generate a reference echo with a predetermined characteristic signal; The signal processor is configured to perform the following operations: Identify a reference echo with a predetermined characteristic signal from the fixed-distance reflector; The actual propagation speed of radar waves in the current gaseous medium is calculated based on the reference echo. The liquid level height is calculated and output using the actual propagation speed.

Images