CN106840310B - Continuous frequency modulation wave radar level gauge measuring method - Google Patents
Continuous frequency modulation wave radar level gauge measuring method Download PDFInfo
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Abstract
The invention discloses a method for measuring a radar level gauge by using continuous frequency modulation waves, which adopts a Chirp-Z + FFT method and combines an automatic gain control method to realize the measurement of the water level under the conditions of large range and large variability and has the characteristics of high precision, high stability, low power consumption, strong anti-interference capability, strong repeatability and the like. The invention can be independently used for high-precision water level measurement, a sensor can be used, and an intelligent integrated monitoring device can be formed by the invention and an acquisition terminal to realize water level early warning and unattended automatic water level monitoring.
Description
The technical field is as follows:
the invention belongs to the technical field of liquid level observation, and relates to a measuring method of a continuous frequency-modulated wave radar level gauge.
Background art:
at present, the domestic radar water level gauge for wide-range and large-variability water level measurement mainly depends on import, mainly takes German brand as main material, and is expensive. Most of domestic radar level gauges are obtained by modifying industrial level gauges, and the main method adopted by the domestic radar level gauges is a pulse radar system which realizes water level measurement by using a time difference method. According to modern radar theory, the resolution of radar ranging is proportional to the bandwidth or time width of the transmitted signal. Due to the limitation of transmitting power, the method cannot obtain larger time width and bandwidth at the same time, when the water level is measured in a large range and the water level variability is larger, because the reflected signal becomes very weak, phase information can be submerged in noise, the measurement precision and stability of the large-range water level are influenced, the measurement precision is mostly 1.0cm or 2.0cm, and the accuracy and stability requirements of the large-range large-amplitude water level measurement cannot be met.
In order to overcome the problems, technical improvement is needed to ensure that the measurement precision and the stability of the domestic radar level gauge reach or exceed the level of the imported radar level gauge.
The invention content is as follows:
the invention designs a measuring method of a continuous frequency modulation wave radar water level meter aiming at the state of the prior art, the method is based on a modulation mode of the continuous frequency modulation radar, the water level measurement under the conditions of large range and large variability is realized by combining a Chirp-Z + FFT algorithm with an automatic gain control method, and the method has the characteristics of high precision, high stability and low power consumption.
In order to achieve the purpose, the invention adopts the following specific technical scheme:
a method for measuring a radar level gauge by using continuous frequency modulation waves mainly comprises a main control module, a radar modulation wave transmitting module, two radar signal receiving modules, an automatic gain control module and a planar microstrip antenna, wherein the two radar signal receiving modules are arranged to form two receiving channels, and the method mainly comprises the following steps:
(1) before the water level measurement of the system, firstly setting gain scale factors of a first-stage amplifier and a second-stage amplifier of two receiving channels of an automatic gain control module;
(2) the radar modulation wave transmitting module continuously transmits a section of radar modulation wave signal A1, a continuous frequency modulation wave frequency sweep signal S1 for detecting the water level is generated through a planar microstrip antenna, the continuous frequency modulation wave frequency sweep signal S1 is reflected by the water surface to be detected to form a microwave echo signal R1, and the echo signal R1 is received and then respectively input into two radar signal receiving modules;
(3) by radar signalsReceiving single-processed orthogonal discrete digital signals x output two groups (same amplitude, equal frequency and 90-degree phase difference)1(nTs) And x2(nTs) The master control module is provided;
(4) the main control module converts the two groups of discrete digital signals into complex number x (nT) according to the following formulas) Form, then pair x (nT)s) Performing FFT (fast Fourier transform), solving a frequency value corresponding to the maximum frequency spectrum point of each signal, and eliminating useless frequency spectrum points through module solving to obtain corresponding water level distance frequency spectrum data and obtain a maximum frequency spectrum point;
x(nTs)＝x1(nTs)+j x2(nTs)
wherein T issRepresenting the sampling period, n represents the number of sampling points, and j represents the imaginary part of the complex number;
(5) the water level distance spectrum data are subjected to multiple accumulation processing by a coherent accumulation algorithm, and the water level distance spectrum data average value of each signal maximum spectrum point is obtained; comparing with a threshold, eliminating bad values smaller than the threshold, and further obtaining a frequency value of the maximum frequency spectrum point;
(6) performing Chirp-Z conversion around the maximum frequency spectrum point obtained by the coherent accumulation algorithm through a Chirp-Z fitting algorithm, and performing fitting processing to obtain a frequency value corresponding to the real water level;
(7) and converting the frequency spectrum value corresponding to the real water level value into a water level value through a frequency spectrum conversion formula, wherein the water level value can be output as an actual measurement value.
The invention is further designed in that:
and (5) taking the obtained 4-time value of the water level distance spectrum data average value of each signal maximum spectrum point as a threshold value.
In the step (5), the number of times of accumulation of the coherent accumulation algorithm is set to 100 times.
The frequency spectrum value corresponding to the real water level in the step (6) is determined by the following method: taking the maximum frequency spectrum point calculated by the coherent accumulation algorithm as the position of a main lobe, taking M points around the adjacent position of the main lobe to carry out Chirp-Z conversion to obtain corresponding extreme points Zr(r-0, 1, …, M-1) spectrum X (z)r)；|X(zr) The frequency value corresponding to the maximum value of | corresponds to the real water level value. And M is 9, and the absolute error of the measurement of the water level is less than 0.1%.
In the step (7), the output of the measured values is accumulated for N times, and the average value is taken as the real water level value. The above N is taken to be 100, and takes about 20 s.
Compared with the prior art, the invention has the following advantages:
1. the method of the invention realizes the measurement of the water level under the conditions of large range and large variability by adopting a Chirp-Z + FFT method and combining an automatic gain control method, and has the characteristics of high precision, high stability, low power consumption, strong anti-interference capability, strong repeatability and the like.
2. The invention can be independently used for high-precision water level measurement, a sensor can be used, and an intelligent integrated monitoring device can be formed by the invention and an acquisition terminal to realize water level early warning and unattended automatic water level monitoring.
Description of the drawings:
fig. 1 is a schematic circuit diagram of the present invention.
Fig. 2 is a schematic diagram of a water level gauge measurement.
FIG. 3 is a flow chart of the master control of the present invention.
Fig. 4 is a flow chart of the coherent accumulation algorithm of the present invention.
FIG. 5 is a flow chart of water level conversion calculation according to the present invention.
Fig. 6 is a flow diagram of an automatic gain control module.
The specific implementation mode is as follows:
the control process and the hardware structure schematic diagram of the present invention are described in detail below with reference to the accompanying drawings.
The first embodiment is as follows:
as shown in FIG. 1, the hardware design of the continuous frequency modulation wave radar level gauge measurement of the invention is as follows:
the radar antenna mainly comprises a main control module, a radar modulation wave transmitting module, two paths of radar signal receiving modules, an automatic gain control module of the two paths of radar signal receiving modules and a planar microstrip antenna.
The radar modulation wave transmitting module comprises a (DDS) triangular wave generator, a (MOD) modulator, a (VCO) voltage-controlled oscillator and a (DISP) circulator which are sequentially connected; the radar signal receiving module comprises a (MIX) high-frequency mixer, (AMP) primary amplifier, (LPF) low-pass filter, (AMP) secondary amplifier and an A/D converter which are connected in sequence; the main control module adopts a TMS320F28M35 dual-core processor and comprises a DSP processor, a CPU processor, an FPU floating point arithmetic unit, a program memory FLASH and an RAM memory.
The main control module is respectively connected with the radar modulation wave transmitting module and the radar signal receiving module; the transmitting end of the planar microstrip antenna is connected with a circulator of a radar modulation wave transmitting module, the receiving end of the planar microstrip antenna is connected with a high-frequency mixer of a radar signal receiving module, the output end of the circulator is also connected with the input ends of the high-frequency mixers of the two paths of radar signal receiving modules respectively, each radar signal receiving module is provided with an automatic gain control module, and the automatic gain control modules are respectively connected with a first-stage amplifier and a second-stage amplifier.
In the invention, two groups of radar signal receiving modules are arranged in parallel to form two channels. The input ends of the two groups of radar signal receiving modules are respectively connected with the receiving ends of the planar microstrip antennas, the output ends of the two groups of radar signal receiving modules are respectively connected with the main control module, and the output ends of the circulators are respectively connected with the two groups of high-frequency mixers. The two paths of orthogonal signals with the same amplitude, the same frequency and the phase difference of 90 degrees are output.
The control flow of the invention is mainly as follows:
as shown in fig. 3, the main control module calls and controls generation of the linear triangular modulation wave, starts a/D acquisition to complete acquisition of the difference frequency signal, and the acquisition period and frequency are programmable; the main control module CPU calls the FFT module to transform the acquired orthogonal two-channel difference frequency signals, calculates frequency values corresponding to the maximum frequency spectrum points, calls the frequency spectrum calculation module, eliminates the frequency spectrum points of useless targets and calculates corresponding water level distance spectrums; in practice, the inherent frequency spectrum leakage effect and frequency spectrum superposition effect influence of FFT (fast Fourier transform) conversion and the white noise influence in the signal transmission process are considered, and the distance spectrum corresponding to the maximum frequency spectrum possibly deviates from a real distance value; in order to reduce measurement errors and improve detection precision, the invention designs and adopts a water level detection coherent accumulation algorithm and a Chirp-Z fitting algorithm; the coherent accumulation algorithm improves the signal-to-noise ratio through N times of accumulation, and basically eliminates the influence of white noise; performing Chirp-Z transformation on the Chirp-Z fitting algorithm around the maximum frequency spectrum point obtained by the coherent accumulation algorithm and performing fitting processing to obtain a frequency value corresponding to the real water level; the water level value is converted by the spectrum conversion formula and can be output as an actual measurement value.
Example two:
the radar positioning principle is shown in fig. 2, wherein a dark line of an F coordinate represents a triangular wave transmitting signal, and a light line represents an intermediate frequency receiving signal after detection by a receiver; fbThe coordinate represents the transceiving difference frequency signal, and it can be seen that the transceiving frequency has a difference frequency F due to the existence of the radio frequency propagation delaybThis frequency value varies in direct proportion to the target distance;
if f is usedbupThe frequency difference (up-sweep frequency difference for short), f, of the radar signal is shown in the ascending period of the triangular wave sweep frequency in fig. 2bdownThe frequency difference (called down sweep frequency difference for short) of the receiving and sending radio frequency of the descending section of the triangular wave sweep frequency in fig. 2 is shown; according to the modern radar theory, the frequency difference f of the upper sweep frequency can be provedbupSum-and-down swept frequency difference fbdownThe following relationships exist:
Fb＝fbup-fbdown＝(4μR)/C+(2vμT)/C (1)
in the formula: t is the frequency modulation period, mu is the frequency modulation slope which is a constant, C is the light speed factor which is also a constant, and R is the distance from the target to the plane of the radar antenna; in the formula, the first term (4 μ R)/C represents the frequency generated by the distance delay, the second term (2v μ T)/C represents the distance of the radial movement vT of the target moving at the v speed in the sweep-down frequency band, the water level change (less than 6m/s) is very small compared with the light speed, and the second term is negligible, so that the target distance is obtained:
R＝(C/4μ)Fb＝(CT/4B)Fb(2)
where B identifies the frequency bandwidth of the sweep.
The invention adopts an optimization algorithm design and obtains an accurate water level measurement value through accurate detection Fb.
Example three:
as shown in fig. 3, the method for measuring a continuous frequency modulation wave radar level gauge of the present invention mainly comprises a main control module, a radar modulation wave transmitting module, a radar signal receiving module, an automatic gain control module and a planar microstrip antenna, and the method mainly comprises the following steps:
step 1), before the actual water level of a main control module is measured, firstly calling an automatic gain control module (the module can be realized by software), setting the tracking times (time) and the tracking step length according to the optimal principle and the real-time requirement, and outputting a digital gain scale factor; controlling the gains of the first-stage amplifier and the second-stage amplifier of the two receiving channels to increase the gain when the echo signal is small; when the signal is large, the gain is reduced; and the signal is maintained to work in an operational amplifier linear region and response speed in a wide range, and the accuracy and stability of the water level measurement of the system are ensured. And after the gain of the operational amplifier is set, the actual water level monitoring is started.
In the invention, a digital negative feedback type automatic gain control module is designed and adopted in a receiving module in a mode of combining hardware and software, the gain of a receiving operational amplifier can be automatically adjusted according to the size of an input signal, and the problem of large-range large-amplitude high-precision water level detection is well solved, and the method comprises the following steps:
before actual water level measurement, calling an automatic gain control module, setting the maximum tracking frequency Nmax (time) and the maximum amplitude Vomax allowed to be output by an operational amplifier according to an optimal principle and real-time requirements, setting the maximum digital gain value Kmax allowed by the automatic gain control module, setting the tracked Step as Kmax/Nmax, and outputting a digitized gain scale factor K;
as shown in fig. 6, during initialization, firstly, the echo signal of the receiving channel is sampled by the ADC, then the FFT algorithm is called to estimate the preliminary water level value, and the preliminary water level value is converted into the output value Vo of the operational amplifier, where the amplitude reflects the size of the water level. When the signal is small, namely the operational amplifier output Vo is smaller than the maximum amplitude Vomax, increasing the gain of the first-stage and/or second-stage amplifier according to the maximum digital gain value and the tracking step; when the signal is too large, namely the operational amplifier output is larger than the maximum amplitude Vomax, the gain of the first-stage and/or second-stage amplifier is reduced according to the maximum digital gain value and the tracking step; the appropriate gain scale factor is thus determined. And the signal is maintained to work in an operational amplifier linear region and response speed in a wide range, and the accuracy and stability of the water level measurement of the system are ensured.
The method can realize the water level monitoring of the amplitude of 0-35m, and the error is less than 2 mm; when the water level variability is more than 40cm/min and less than 100cm/min, the stable measurement of the water level can be realized.
Step 2), the radar modulated wave transmitting module continuously transmits a section (with adjustable period and amplitude) of radar modulated wave signal A1, a continuous frequency-modulated wave frequency-sweep signal S1 for detecting the water level is generated through the planar microstrip antenna, the continuous frequency-modulated wave frequency-sweep signal S1 is reflected by the water surface to be detected to form a microwave echo signal R1, and the microwave echo signal R1 is received and then input to the radar signal receiving module;
and step 3), the two paths of radar signal receiving modules are used for processing, high-frequency carriers are eliminated after frequency mixing detection and phase shifting processing are carried out, two orthogonal difference frequency analog signals IQ and IF are sent out, the difference frequency signals IQ and IF are orthogonal signals with the same amplitude, the same frequency and 90-degree phase difference, and complex signals required by subsequent DFT conversion are naturally formed so as to realize rapid operation of frequency spectrums. IQ and IF are processed by first-stage operational amplifier, low-pass filter and second-stage operational amplifier and then sent to AD converter of the latter stage, two paths of difference frequency analog signals are A/D converted and then discrete digital signals x are output respectively1(nTs) And x2(nTs) The master control module is provided;
where N represents the number of sample sequences (0,1 … … N-1), TsRepresents a sampling period; n represents the maximum number of sample points, in this example 2048 points.
Step 4), the main control module enables the two groups of discrete digital signals x to be transmitted1(nTs) And x2(nTs) Conversion to complex form x (nT)s) Then on x (nT)s) Performing FFT (fast Fourier transform) to obtain a frequency value corresponding to the maximum frequency spectrum point of each signal, and then eliminating the frequency spectrum points of useless targets through a frequency spectrum module to obtain corresponding frequency spectrum data and obtain the maximum frequency spectrum point;
the main control module converts the data into a plurality of data, namely: x (nT)s)＝x1(nTs)+j x2(nTs)。
Wherein, TsRepresenting the sampling period, N represents the number of sample sequences (0,1 … … N-1), and j represents the imaginary part of the complex number;
discrete digital signal x (nT)s) To TsAssuming that 1 is set after the normalization process, the fourier transform (DFT) of the discrete sequence x (n) can be expressed as follows:
wherein X (k) represents the frequency spectrum of the difference signal, WN＝e-j2π/NRepresents a disc factor; k represents a sampling point, k is 0,1, …, N/2; obtaining a frequency spectrum by using fast FFT (fast Fourier transform) on the formula (3);
the spectrum a (k) is obtained according to the following formula:
A(k)＝1/N(ReX(k)2+ImX(k)2) (4)
in formula (4), a (k) is a spectral function of a sample point k; n represents the maximum number of sampling points;
according to fk＝kfsObtaining the frequency of each point of corresponding sampling points k which is 0,1, … and N/2;
wherein f issIs the sampling frequency, and TsThe sampling period is corresponding, and N is the maximum sampling point number;
as known from modern radar theory, the maximum of the frequency spectrum is fk＝FbAnd (3) corresponding to the distance point of the target, and directly calculating a water level value R corresponding to the maximum frequency spectrum according to the formula (2).
R＝(C/4μ)Fb＝(CT/4B)Fb(2)
And step 5), carrying out multiple accumulation processing on the frequency spectrum data through a coherent accumulation algorithm, and basically eliminating the inherent frequency spectrum leakage effect of FFT (fast Fourier transform) and the white noise influence in the signal transmission process.
As shown in fig. 4, the invention designs a coherent accumulation algorithm for water level detection to reduce the influence of noise on the accuracy of water level detection, and the coherent accumulation algorithm essentially performs N/2-point sampling on equation (4) to obtain a sample value a (k) (k is 0,1 … N/2-1) of a frequency spectrum, accumulates M times corresponding to each k-point value, and obtains an average value after accumulation, thereby improving the signal-to-noise ratio of a signal. The concrete implementation is as follows:
the module initializes the accumulation number N as 0, the frequency spectrum accumulation quantity L (k) as 0, the module accumulates each frequency spectrum sampling point for M times, the value of M can be set according to actual needs, the item takes M as 100, the module judges whether the accumulation process is finished, the module calculates the average power of each accumulated point frequency spectrum value, and sets the average power of 4 times as a judgment threshold value, and judges whether the distance target exists or exceeds the detection range. And eliminating bad values smaller than a threshold according to the judgment threshold value, and obtaining the frequency value of the maximum spectrum point of the accumulated frequency spectrum, wherein the signal-to-noise ratio of the frequency value is improved by M times, and the measurement error introduced by noise is reduced by M times.
Step 6), performing M-point Chirp-Z transformation around the maximum spectrum point by a Chirp-Z fitting algorithm, and performing fitting treatment to obtain a frequency value corresponding to a more accurate water level;
the invention designs frequency measurement errors caused by the inherent frequency spectrum leakage effect and frequency spectrum superposition effect of FFT conversion and adopts a Chirp-Z conversion software algorithm, and the realization process is as follows:
the spectrum (i.e. distance spectrum) obtained by FFT is a digital spectrum, which is represented as a distance sampling interval fixed on the distance axis, the distance interval can be reduced by increasing the number of sampling points N, the water level measurement precision is improved, the value of N cannot be too large due to the limit of the actual digital signal processing level and the operation speed, so that a certain sampling interval always exists on the actual radar distance spectrum, and the spectrum obtained by directly adopting FFT has a fixed sampling interval △ f and a fixed sampling interval DeltaR corresponding to the distance domain due to the fence effect and the spectrum superposition effect of FFT, so that the distance measurement error with the maximum DeltaR/2 can be generated.
In order to improve the frequency estimation precision and further improve the water level detection precision, the invention adopts a Chirp-Z transformation algorithm. The method can obviously improve the detection precision of the water level without influencing the operation speed of the detection system under the condition that the number of FFT points is not increased greatly.
The method comprises the following steps: according to the maximum spectrum point calculated by a coherent accumulation algorithm, namely the position of a main lobe, M points are taken around the adjacent position of the main lobe to carry out Chirp-Z transformation, and the algorithm is as follows:
Wherein A, W is a related complex parameter,
CZT denotes Chirp-Z transformation, theta0Is the starting argument phi 02 pi/N is the argument increment.
In this example, take A0＝W0X (Z) of Chirp-Z conversion of the above formula when 1 is truer) I.e. the corresponding extreme point Z on the Z-plane unit circler(r ═ 0,1, …, M-1); i X (z)r) The maximum value of i corresponds to a more accurate frequency value, i.e. to the true water level value. Simulation tests show that the detection precision can be improved by 1-2 orders of magnitude by taking a proper M value and giving consideration to the requirements of real-time property and detection precision. The invention takes M to 9, so that the absolute error of water level detection is less than 0.1%.
And 7), converting the obtained frequency value into an accurate water level value by using a water level conversion module, as shown in fig. 5. The water level value is output in a digital form through RS485, and meanwhile, a water level signal of 4-20mA is output in an analog quantity form.
FIG. 5 is an algorithm flow for the water level conversion module, which utilizes | X (z) as described abover) And (4) taking the frequency value corresponding to the maximum value of | as an accurate frequency value, and converting the frequency into a water level value by using the formula (2), wherein the water level value can be used as an actual measurement value.
In practice, to take into account the effect of waves or surface floats on the measurements, these effects can also be eliminated by means of a mean algorithm of distances. The module finishes the accumulation of ranging values in a certain time, and can set the value of the accumulation times N according to the requirement. The water level conversion module calculates the average value of the distance N points as a real water level value to be measured, and outputs the measured water level value to the user side through the RS485 communication interface. R ═ C/4. mu. Fb＝(CT/4B)Fb(2)
The invention takes about 20 seconds when taking N as 100 times, and the influence of waves and water surface floaters on the water level detection is basically eliminated through simulation verification.
Test example one:
the test method of the engineering prototype, the test waveform and the detection data table are as follows: the water level is simulated by using a surface target mounted on a movable scale, and the data measured by a laser range finder is used as a true value. And repeating the test for multiple times, wherein during the resolution detection, the surface target is moved by 50mm according to the step pitch within the range of different water levels of 5m, 10m, 20m and 30m and 0.4m, and the measurement data of 10m and 20m repeated for 5 times are selected and listed to obtain the maximum absolute error and the average error. And carrying out accuracy detection under the conditions that the range of range change is 0-20 m and the water level variability is 1 m/min.
In fig. 2, a modulated triangular wave is arranged at the upper part, the rising edge of the triangular wave is an upper sweep frequency wave, and the falling edge of the triangular wave is a lower sweep frequency wave; the difference frequency wave is received by the upper frequency sweep and the lower frequency sweep at the lower part of the figure 2.
TABLE 1 Water level 10m step 50mm quintic detecting table
TABLE 2 Water level 20m step 50mm quintic detecting table
Multiple detection results show that the maximum absolute error is 2mm when the range of 0-20 m is within, the average error is not more than 1mm, and the resolution is better than 1 mm.
The invention has the main performance and technical indexes that:
(1) and (3) signal output: RS485+4-20mA output.
(2) Range of measurement: 0.8 to 35 m.
(3) Stability: < 0.2% FS/year.
(4) Composite error (including linearity, hysteresis, repeatability): 0.2% FS.
(5) And (3) measuring precision: +/-3 mm (0.8-35 m)
(6) Measuring time: 20s
(7) The beam angle is less than or equal to 10 °
(8) Power supply: 9-16VDC
(9) Power consumption: 5mA
(10) Ambient temperature: -25 ° - +60 ° -%
(11) Ambient humidity: 0-90H
(12) Insulating strength: UDC 500V ≥ 200M Ω
(13) Protection grade: IP 55.
Claims (7)
1. A method for measuring a radar level gauge by using continuous frequency modulation waves mainly comprises a main control module, a radar modulation wave transmitting module, two radar signal receiving modules, an automatic gain control module and a planar microstrip antenna, wherein the two radar signal receiving modules are arranged to form two receiving channels, and the method mainly comprises the following steps:
(1) before the water level measurement of the system, firstly setting gain scale factors of a first-stage amplifier and a second-stage amplifier of two receiving channels of an automatic gain control module;
(2) the radar modulation wave transmitting module continuously transmits a section of radar modulation wave signal A1, a continuous frequency modulation wave frequency sweep signal S1 for detecting the water level is generated through a planar microstrip antenna, the continuous frequency modulation wave frequency sweep signal S1 is reflected by the water surface to be detected to form a microwave echo signal R1, and the microwave echo signal R1 is received and then respectively input into two radar signal receiving modules;
(3) after radar signal receiving single processing, two groups of orthogonal discrete digital signals x with same amplitude, equal frequency and 90-degree phase difference are output1(nTs) And x2(nTs) The master control module is provided;
(4) the main control module converts the two groups of discrete digital signals into complex number x (nT) according to the following formulas) Form, then pair x (nT)s) Performing FFT fast Fourier transform to obtain maximum frequency spectrum point of each signalThe corresponding frequency value is subjected to modulus calculation, useless spectrum points are removed, corresponding water level distance spectrum data are obtained, and the maximum spectrum point is obtained;
x（nTs） = x1(nTs) +j x2(nTs)
wherein T issRepresenting the sampling period, n represents the number of sampling points, and j represents the imaginary part of the complex number;
(5) the water level distance spectrum data are subjected to multiple accumulation processing by a coherent accumulation algorithm, and the water level distance spectrum data average value of each signal maximum spectrum point is obtained; comparing with a threshold, eliminating bad values smaller than the threshold, and further obtaining a frequency value of the maximum frequency spectrum point;
(6) performing Chirp-Z transformation around the maximum spectrum point obtained by the coherent accumulation algorithm through a Chirp-Z fitting algorithm and performing fitting processing to obtain a spectrum value corresponding to the real water level;
(7) converting the frequency spectrum value corresponding to the real water level value into a water level value through a frequency spectrum conversion formula, wherein the water level value can be output as an actual measurement value;
before the water level measurement, the system firstly sets the gain scale factors of the first-stage amplifier and the second-stage amplifier of the two receiving channels of the automatic gain control module, and comprises the following steps:
before actual water level measurement, calling an automatic gain control module, setting the maximum tracking frequency Nmax and the maximum amplitude Vomax allowed to be output by the operational amplifier, the maximum digital gain value Kmax allowed by the automatic gain control module, the Step = Kmax/Nmax of the tracked Step, and outputting a digitized gain scale factor K according to an optimal principle and real-time requirements;
initializing, namely firstly sampling a microwave echo signal of a receiving channel by an ADC (analog to digital converter), then calling an FFT (fast Fourier transform algorithm) algorithm to estimate a preliminary water level value, converting the preliminary water level value into an operational output value Vo, wherein the amplitude of the operational output value Vo reflects the size of the water level; when the signal is small, namely the output value Vo of the operational amplifier is smaller than the maximum amplitude Vomax, the gain of the first-stage and/or second-stage amplifier is increased according to the maximum digital gain value and the tracking step; when the signal is too large, namely the output value of the operational amplifier is larger than the maximum amplitude Vomax, the gain of the first-stage and/or second-stage amplifier is reduced according to the maximum digital gain value and the tracking step; the appropriate gain scale factor is thus determined.
2. The continuous FM radar level gauge method according to claim 1, wherein in step (5), the threshold value is 4 times of the average value of the water level distance spectrum data of each maximum spectrum point of the signal.
3. The continuous FM wave radar level gauge measuring method according to claim 2, wherein in the step (5), the number of times of accumulation of the coherent accumulation algorithm is set to 100 times.
4. The continuous Frequency Modulation (FMCW) radar level gauging method according to claim 1, 2 or 3, wherein the spectral values corresponding to the true water level in step (6) are determined by: taking the maximum frequency spectrum point calculated by the coherent accumulation algorithm as the position of a main lobe, taking M points around the adjacent position of the main lobe to carry out Chirp-Z conversion to obtain corresponding extreme points ZrSpectrum X (z) ofr) Wherein r = 0,1, …, M-1; i X (z)r) The frequency value corresponding to the maximum value of | corresponds to the real water level value.
5. The continuous FM radar level gauge measuring method as claimed in claim 4, wherein M is 9, and the absolute error of the water level measurement is less than 0.1%.
6. The continuous FM radar level gauge measuring method as claimed in claim 4, wherein in the step (7), the measured value output is accumulated N times, and the average value is taken as the true level value.
7. The continuous frequency modulated wave radar level gauge measuring method according to claim 6, wherein N is 100.
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| CN110763302A (en) * | 2019-11-20 | 2020-02-07 | 北京航空航天大学 | FMCW high-precision liquid level measurement method based on iterative frequency estimation |
| CN113534145A (en) * | 2021-09-16 | 2021-10-22 | 北京安达维尔航空设备有限公司 | Altimeter height measurement method and system based on linear frequency modulation continuous wave system |
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