CN1632474A - Nonlinear Evolutionary Correction Method for LFM Radar Level Meter - Google Patents

Abstract

This invention relates to a non-linear correction method of linear frequency-modulated radar object meter, which belongs to signal process and automatic measurement technique field. It divides the frequency-scanning voltage curve of the control oscillator and uses frequency spectrum analysis to extract characteristics information and adopts principle of evolution computation to get the end point coordinates of the curve to form non-linear frequency-scanning curve to compensate the non-linear property of the control oscillator without measurement of high-frequency microwave signals or phase to generate the linear frequency-modulated signals with high linear property. It solves the problem of linear frequency-modulated radar non-linear property.

Description

Nonlinear Evolutionary Correction Method for LFM Radar Level Meter

1. Technical field

The non-linear evolution correction method of the chirp radar level gauge of the present invention belongs to the technical field of signal processing and automatic measurement, specifically, it uses spectrum analysis to extract characteristic information and adopts the principle of evolutionary calculation to solve the problem of chirp radar frequency modulation nonlinearity.

2. Technical Background

The linear continuous frequency modulation radar level gauge has the advantages of non-contact, high resolution and wide media adaptability in material level measurement, and is widely used in various material level measurements. Since the voltage-controlled oscillator used in the linear continuous wave FM radar is inherently nonlinear, when a completely linear voltage is used for frequency modulation, the final output frequency signal will exhibit nonlinear characteristics. Due to the frequency offset, the frequency mixing The final beat signal is not a single-frequency signal in the case of an ideal linear frequency modulation signal, and the spectrum will be broadened, resulting in a decrease in the resolution of the LFMCW radar level gauge, and a decrease in the signal-to-noise ratio after FFT spectrum analysis, affecting the final calculation accuracy. In order to eliminate nonlinearity, two methods are generally used: open-loop correction and closed-loop correction. The open-loop correction is to measure the voltage-frequency relationship of the VCO in advance, store it in the memory, and output the frequency-sweeping voltage by the D/A converter according to the digital quantity stored in advance in actual use. Or use potentiometers and amplifiers to form a multi-stage non-linear correction hardware circuit for hardware correction. The disadvantage of these two methods is that the characteristics of each VCO are different, and the workload in the production and debugging process is very large, and it is not easy to batch. Production. It is also greatly affected by temperature in the actual use process, and it is difficult to guarantee the measurement accuracy. The method of closed-loop correction is to use the frequency or phase measurement method to find the frequency under the corresponding voltage, and then correct the digital quantity that generates the control voltage to obtain a satisfactory linearity, which will increase the hardware cost and software workload.

In order to solve this problem, people use the method of direct digital synthesis and digital phase-locked loop to digitally synthesize the required signals, but they cannot be put into practical application due to the following reasons. First of all, the method of direct digital synthesis is still not up to the X-band of the material level radar, and only the method of frequency doubling or up-conversion can be used. This method is costly and the system is complicated. Second, it is generated by frequency doubling. Microwave signals have many harmonic components and large phase noise, which will have a negative impact on the measurement results. In addition, a transition process is required when changing the frequency with the phase-locked loop, so the frequency conversion time is relatively long, which cannot meet the requirements of the high-speed frequency-sweeping characteristic of the chirp radar.

3. Contents of the invention

The purpose of the nonlinear evolution correction method of the linear frequency modulation radar level meter of the present invention is to provide a simple correction method of frequency modulation linearity without adding any hardware, to solve the current difficult and complicated problem of linearity correction, and to solve the problems caused by the non-linearity of frequency modulation. To solve the problem of the influence of linearity on measurement accuracy, a radar level gauge that is easy to produce and debug is provided.

The linearity evolution correction method of the LFMCW radar level gauge voltage-controlled oscillator of the present invention is characterized in that: utilize the evolutionary principle to correct the frequency modulation nonlinearity of the voltage-controlled oscillator, 1. adopt a kind of new method based on evolution Oscillator frequency nonlinearity for closed-loop correction principle; II. Use the nonlinearity of the frequency modulation voltage curve to compensate for the nonlinearity of the voltage-controlled oscillator, divide the frequency modulation voltage curve into several sections, and divide the coordinates of each end point into a certain change area. The principle of evolution obtains the coordinates of the endpoints corresponding to the best frequency modulation linearity, and connects the endpoints to form a frequency sweep voltage curve; III. Using the method based on the analysis of the frequency spectrum characteristics of the difference frequency signal for correction, there is no need to measure the frequency or frequency of high-frequency microwave signals Phase, the existing radar level gauge can be corrected without adding any new hardware equipment, or adding a delay in the microwave unit can realize online self-calibration; IV. The method of overlapping analysis can be used to obtain higher correction linearity Specifically, the hardware basis of the linearity evolution correction method of the LFMCW radar level gauge voltage-controlled oscillator of the present invention mainly includes a digital signal processing unit and a microwave unit [see Fig. 1 for structure details], and the digital signal processing unit is displayed by a keyboard Composed of communication interface module 1, D/A converter 2, digital signal processor 3, buffer amplifier 4, memory 5, filter 12, digital program-controlled gain amplifier 13 and high-speed A/D converter 14, the microwave unit is composed of voltage-controlled oscillation Composed of 6, coupler 7, antenna 9, circulator 10 and mixer 11, the digital signal processor 3 outputs the digital signal to the D/A converter 2, converts it into a voltage signal, and forms the microwave unit through the buffer amplifier 4 The control voltage of the voltage-controlled oscillator 6 excites the voltage-controlled oscillator 6 to generate a microwave frequency modulation signal, and the microwave frequency modulation signal passes through the coupler 7 and then is emitted by the antenna 9 through the circulator 10, and the signal coupled out by the coupler 7 at the same time is sent as a reference signal to the mixer 11, the echo reflected by the target object 8 is received by the antenna 9, and after passing through the circulator 10, it is also sent to the mixer 11 to mix with the reference signal, and the mixed signal passes through the low-pass filter 12 to form a difference frequency The signal and the difference frequency signal are sampled by the high-speed A/D converter 14 after passing through the digital program-controlled gain amplifier 13, converted into a digital sequence, read by the digital signal processor 3, and stored in the memory 5, and the digital signal processor 3 samples the digital sequence Carry out FFT or wavelet calculation and analysis to extract the characteristic quantity representing nonlinearity;

There is an essential nonlinearity in the voltage controlled oscillator [see Fig. 3]. In Fig. 3, the ideal FM transmission signal 18 and the ideal echo signal 20 are mixed to form a point frequency signal, and the actual FM transmission signal 17 has nonlinear characteristics. The actual echo signal 19 also has nonlinear characteristics. After the two are mixed together, the frequency spectrum will be aliased, which will reduce the resolution of the measurement or make it impossible to distinguish the position of the specific target object. The spectral peak 21 in Fig. 4 is corresponding to the specific Material level, there are two peak points 22 and 23 in very close positions, which may lead to the inability to distinguish the specific material position. This is a relatively good situation measured in the laboratory, and it will be more accurate in the actual measurement process. Seriously, Fig. 5 is the frequency spectrum after the linearity is corrected by the method of the present invention, except for the spectral peak 24 representing the material level, other components are effectively suppressed, and the influence of the phase noise and nonlinearity of the VCO is eliminated , the nonlinear evolution correction method of the voltage-controlled oscillator of the linear frequency modulation radar level meter of the present invention adopts the principle based on the evolution method to carry out segmental linear correction to the frequency modulation voltage curve, and the specific method is to divide the frequency modulation voltage curve into several sections, as shown in Figure 6 The FM curve is divided into six segments, which are line segment 25, line segment 26, line segment 27, line segment 28, line segment 29, and line segment 30. There are seven endpoints in total, which are the starting point 31, the ending point 37, and the middle variable area allocated with variable regions. Change the endpoints 32, 33, 34, 35, 36, the starting point 31 and the ending point 37 are fixed points, connect the endpoints to form a nonlinear frequency modulation voltage curve, and compensate the nonlinearity of the voltage-controlled oscillator. The end point of the end point is obtained by an evolutionary method. By analyzing the frequency spectrum after FFT or wavelet operation, the characteristic quantity representing nonlinear and interference information is obtained. The spectral peak 21 in Figure 4 corresponds to the specific material level, which is required in the measurement process. , and the other two peak points 22 and 23 are caused by the phase noise and nonlinearity of the VCO, which need to be suppressed, and the sum of several peak quotients within a certain range on both sides of the spectrum peak can be selected as the evaluation of the nonlinearity caused by The evaluation function of the influence, by making the function reach a certain value through the evolution method, the influence caused by the phase noise of the voltage-controlled oscillator and the nonlinearity of frequency modulation can be effectively suppressed, for example, the value of the spectrum peak 21 and the spectrum peak 22 in Figure 4 can be selected , The sum of the quotient of 23 numerical values is used as evaluation function, to the evaluation of evolutionary correction effect, the method of evolutionary correction is shown in Fig. 9 for details, and the first sub-process 43 after the correction process starts in Fig. 9 is used for changing in each intermediate variable endpoint Randomly generate a coded population with intermediate variable endpoint coordinates in the region. The code can be binary coded or other coded forms. The number of coded digits can be determined as needed. The number of individuals in the group is N. The quantity of is M, is decided by the quantity of curve segmentation, and the subprocess 44 thereafter is cleared to loop variable, and subprocess 45 converts the coding value of each endpoint of the curve in the individual into a coordinate value, and subprocess 46 converts the starting point, The intermediate variable endpoints and termination points are connected together to generate a frequency sweep voltage curve, which is actually a digital sequence, and the sub-process 47 continuously outputs the digital sequence at a predetermined rate through the D/A converter 2 to generate For the frequency-sweeping voltage curve of the voltage-controlled oscillator 6, the voltage-controlled oscillator 6 is controlled to output a microwave frequency modulation signal, which is transmitted to a flat metal sheet through the coupler 7, the circulator 10, and the antenna 9, and the reflected echo signal received by the antenna 9 is passed through The circulator 10 enters the mixer 11 and mixes the reference frequency signal coupled through the coupler 7, and the generated mixed frequency signal passes through the filter 12 to form a difference frequency signal, and after being amplified or attenuated by the digital program-controlled gain amplifier 13, the In the sub-process 48, the high-speed A/D converter 14 carries out sampling and quantization to form the digital sequence of the difference frequency signal. In the sub-process 49, the digital signal processor 3 performs FFT operation or wavelet analysis on the sampled digital sequence to obtain a spectrum curve, and perform The feature quantity extraction of the curve, the subsequent sub-process 50 is used to calculate the performance evaluation function of this frequency-sweeping voltage curve, and the sub-process 51 judges whether the evaluation function meets the predetermined requirements, and if the requirements are met, the result is saved by the sub-process 52, and the correction is completed process, otherwise the sub-processes 53 and 54 increment and judge the cyclic variables to ensure that the curve represented by each individual in the coding population can be tested and evaluated, and the evolution calculation sub-process 55 performs evolution after the initial population testing and evaluation is completed Calculate, the resulting group of results repeats the above-mentioned test and evaluation process, the thick line sweep voltage curve 38 in Figure 7 represents the corrected sweep voltage curve, it can be seen that the sweep voltage curve 38 has been nonlinear , modify the nonlinearity of the voltage-controlled oscillator to obtain a microwave FM signal with high linearity through the nonlinearity of the frequency-sweeping voltage curve 38. It should be noted that in the sub-process 55, a genetic algorithm or other evolutionary algorithms such as thinking evolution, Evolutionary programming, immune evolution methods, etc., and the selection of evaluation functions is also diversified. In the present invention, the frequency modulation voltage curve is divided into 6 sections, and it is also feasible to use other numbers of sections. The more sections there are, the better the correction effect will be. Increase the sampling rate of A/D conversion to obtain a sampling sequence exceeding the number of FFT points, and use the overlapping FFT operation on the original sampling sequence to obtain multiple spectra of a single sweep curve. Figure 8 shows in detail the overlapping analysis of the original sampling sequence Segment thinking, for example, can be divided into 39, 40, 41, and 42 data segments of the same overlapping sequence length, perform FFT operation on each segment, and evaluate the feature quantities on multiple spectrums to obtain correction results with higher linearity , but it will increase the amount of computation.

The invention has the advantages that it is mainly used for the measurement of various material levels in industrial and mining enterprises and can be used for some distance measurement occasions. The calibration of chirp linearity can be done by the level meter itself, without external measuring equipment and instruments, the calibration method and algorithm are simple, and the linearity after calibration is high; the cost of the instrument is low, the production and debugging are simple, and the production efficiency is improved; the measurement accuracy High, small error, strong anti-interference ability, adapt to harsh industrial environment, stable and reliable.

4. Description of drawings

Fig. 1 is a structural diagram of the present invention's implementation form 1

Fig. 2 is the structural diagram of form 2 of the present invention

Figure 3 is the nonlinear characteristics of the voltage controlled oscillator

Figure 4 Spectrum before correction

Figure 5 Corrected Spectrum

Figure 6 Segmentation of frequency sweep voltage curve

Figure 7 Sweep voltage curve after correction

Figure 8 Overlapping segments of the original sampling sequence

Figure 9 Linearization correction flow chart

Figure 10 The frequency sweep voltage curve corresponding to the evolution result

Label in the figure

1. Display keyboard communication interface 2. High-speed D/A converter

3. Digital signal processor 4. Buffer amplifier

5. Memory 6. Voltage Controlled Oscillator

7. Coupler 8. Target object or metal plate

9. Antenna 10. Circulator

11. Mixer 12. Low-pass filter

13. Digital programmable gain amplifier 14. High-speed A/D converter

15. Microwave delay line 16. Microwave switch

17. The actual transmitted nonlinear signal 18. The ideal linear transmitted signal

19. The actual received echo nonlinear signal 20. The ideal echo signal

21. The target peak in the spectrum curve 22. The interference peak generated by the non-linearity of frequency modulation

23. Interference peak generated by frequency modulation nonlinearity 24. Single peak after evolutionary linear correction

25. Frequency sweep voltage curve segment 1 26. Frequency sweep voltage curve segment 2

27. Frequency sweep voltage curve segment 3 28. Frequency sweep voltage curve segment 4

29. Frequency sweep voltage curve segment 5 30. Frequency sweep voltage curve segment 6

31. The starting point of the frequency sweep voltage curve 32. The middle variable endpoint 1 of the frequency sweep voltage curve and its

Area of change

33. Variable terminal 2 in the middle of the frequency sweep voltage curve 34. Variable terminal 3 in the middle of the frequency sweep voltage curve and its

and its change area

35. Variable terminal 4 in the middle of the frequency sweep voltage curve 36. Variable terminal 5 in the middle of the frequency sweep voltage curve and its

and its change area

37. End point of frequency sweep voltage curve 38. Corrected frequency sweep voltage curve

39. Original sampling sequence overlapping segment 1 40. Original sampling sequence overlapping segment 2

41. Original sampling sequence overlapping segment 3 42. Original sampling sequence overlapping segment 4

43. Generate the coding group of the endpoint coordinates 44. Clear the loop variable

45. Convert the end point encoding value in the individual into an end point 46. Connect the end points to generate a frequency sweep curve

The coordinate value of

47. Output frequency sweep voltage curve 48. Obtain the sampling digital sequence of the difference frequency signal

49. Perform spectrum calculation and extract spectrum features 50. Calculate the performance of frequency sweep voltage curve

quantity

51. Judging whether the linearity meets the requirements 52. Saving the calibration results

53. Incrementing the loop variable 54. Judging whether the evaluation of each individual in the group is completed

55. Perform evolutionary calculations on populations

5. Specific implementation

Embodiment 1 As shown in Figure 1, during calibration, a metal plate is placed perpendicular to the axial position of the antenna in front of the radar level gauge, and the distance between the radar level gauge and the metal plate can be initially measured and input to the radar level gauge Among the relevant parameters, as a reference, the correction process can be accelerated. The digital signal processor 3 outputs digital signals to the D/A converter 2, converts them into voltage signals, and forms the control of the voltage-controlled oscillator 6 of the microwave unit through the buffer amplifier 4. voltage, to excite the voltage-controlled oscillator 6 to generate a microwave frequency modulation signal, the microwave frequency modulation signal passes through the coupler 7 and then is emitted by the antenna 9 through the circulator 10, and the signal coupled by the coupler 7 is sent to the mixer 11 as a reference signal, and the target The echo reflected by the object 8 is received by the antenna 9, and after passing through the circulator 10, it is also sent to the mixer 11 to mix with the reference signal. The mixed signal passes through the low-pass filter 12 to form a difference frequency signal, and the difference frequency signal passes through the digital After the programmable gain amplifier 13 is sampled by the high-speed A/D converter 14, converted into a digital sequence read by the digital signal processor 3, and stored in the memory 5, the digital signal processor 3 performs FFT operation and analysis on the sampled digital sequence, and then Utilize the correction method of the chirp radar level gauge of evolutionary correction of the present invention to carry out correction by keyboard input command, table 1 has provided the example of an initial group, by 5 intermediate variable endpoint X coordinates and Y coordinates of 6 segments The 16-bit plastic value of the binary code of the initial population, the number of individuals N=35, the number of variables in the individual M=10, Table 2 shows the coded values of the X and Y coordinates of the five intermediate variable endpoints obtained after the evolutionary calculation and the process The converted coordinates, as well as the starting point coordinates (0, 0) and end point coordinates (13383, 13383). Figure 10 is the digital sequence of the control frequency sweep voltage generated by connecting the coordinates of each end point

Table 1 Initial population example serial number variable endpoint 1 variable endpoint 2 variable endpoint 3 variable endpoint 4 variable endpoint 5 X1' Y1' X2' Y2' X3' Y3' X4' Y4' X5' Y5' 1 16838 5202 26703 2750 28528 21293 1696 19975 413 23236 2 5758 9171 6270 879 23363 29441 18585 10519 26281 7254 3 10113 4434 13870 30989 21087 19484 13455 21472 12389 11688 4 17515 28317 11529 30134 19052 12759 92 19641 11357 5003 5 31051 24582 27499 29752 31235 15400 18288 9349 4887 32203 6 5627 6815 4500 28364 15109 6211 31755 23391 17532 8788 7 23010 4586 8607 4880 17075 4144 29167 30563 1555 23915 8 7419 9653 5808 5629 11755 15335 27412 22184 12299 1199 9 16212 26306 15725 2235 10675 22704 25030 19958 9490 13424 10 4086 7174 12457 21332 288 32520 31717 41 26460 24408 11 2749 18451 16542 24145 32053 23789 24842 27666 24911 6490 12 12767 23448 16474 3356 14157 32121 28572 23363 14183 27655 13 9084 6473 11531 5243 5758 21913 26393 25344 22704 21593 14 12060 32434 17222 3079 5222 23571 27828 2410 17412 13310 15 32225 8193 3952 3988 17488 12369 11805 28635 20961 3072 16 17543 14110 17024 807 18945 2770 3660 1006 20382 12038 17 25089 24748 19894 24979 10294 1594 5838 20068 3608 5087 18 21183 28210 24015 31357 11200 887 9046 26660 5811 1697 19 25137 29320 18247 914 5171 18093 18182 20562 7168 23526 20 25566 32049 11276 21187 14305 32317 23772 29323 20794 7448 twenty one 26966 12956 26278 3540 7951 11188 12354 13764 12286 17497 twenty two 4978 14162 19365 14022 6601 27640 15377 21307 13771 15802 twenty three 20495 4166 8746 10149 23608 4295 14885 7773 27844 19649 twenty four 10311 14997 21976 609 7214 12490 22759 10683 6684 30555 25 11367 7793 18092 29009 6377 25859 22934 9013 11255 25891 26 30054 32310 25851 24833 13865 3106 1903 8216 29227 1020 27 17031 21391 29088 16696 25369 24786 6159 28834 26960 18267 28 13145 19799 29163 5432 27215 17097 26145 15571 27448 14366 29 19882 7926 2231 24999 8030 5062 10045 15241 9086 23408 30 25736 14905 26233 28863 177 27943 26847 2303 8526 12848 31 30524 25885 29732 16369 16849 31247 17401 24754 1211 29762 32 28505 2582 21106 28676 11337 12292 29783 928 26131 22042 33 28394 15610 5411 24077 2699 16846 4167 3417 8645 23252 34 22102 5000 9874 7701 23099 3962 781 32048 11552 31440 35 24851 8052 5448 1691 8531 28479 24287 12018 18627 21810

column curve. The frequency sweep curve is output through the D/A converter 2 to obtain a linear FM frequency. After the calibration, you can enter the normal measurement mode and install it on site. Therefore, it is also very convenient to use the method of the present invention to perform calibration on site. In normal use, the real material level signal is calculated and displayed via the display keyboard communication interface unit 1, or output to an external display instrument through current, or communicates with an external display instrument through a communication interface to transmit data;

Table 2 Calibration results starting point variable endpoint 1 variable endpoint 2 variable endpoint 3 variable endpoint 4 variable endpoint 5 end X0 Y0 X1 Y1 X2 Y2 X3 Y3 X4 Y4 X5 Y5 X6 Y6 Integer value corresponding to binary encoding none none 13061 4002 3410 9286 11470 12125 5166 5896 15445 21762 none none coordinate 0 0 2170 2608 4223 4677 6601 6820 8714 9240 11133 10986 13383 13383

Embodiment 2 As shown in Figure 2, a microwave delay line 15 and a microwave switch 16 are added inside the linear frequency modulation radar level gauge of the evolution correction. The microwave delay line 15 produces a certain delay to the microwave, which can simulate a target signal at a fixed distance. Therefore, online self-calibration can be realized. The digital signal processor 3 outputs a digital signal to the D/A converter 2, converts it into a voltage signal, forms the control voltage of the voltage-controlled oscillator 6 of the microwave unit through the buffer amplifier 4, and excites the voltage-controlled oscillator 6 to generate a frequency modulation signal. After the signal passes through the coupler 7, it is emitted by the antenna 9 through the circulator 10. The coupler simultaneously couples two signals, one of which is sent to the mixer 11 as a reference signal, and the other is sent to the microwave switch after a fixed delay through the delay line 15. 16. The target reflection signal received by the antenna 9 is also transmitted to the microwave switch 16 after passing through the circulator 11. The microwave switch 16 is controlled by the digital signal processor 3, and the microwave signal through the delay line 15 and the signal from the circulator 10 Select, allow one of them to pass, the signal passing through the microwave switch 16 is mixed with the reference signal in the mixer 11, the mixed signal passes through the low-pass filter 12 to form a difference frequency signal, and the difference frequency signal passes through the digital program-controlled gain amplifier After 13, it is sampled by the high-speed A/D converter 14, converted into a digital sequence, read by the digital signal processor 3, and stored in the memory 5. When correcting, switch the microwave switch to the state that allows the microwave signal of the delay line 15 to pass through. The method of the present invention can be used for correction. During normal measurement, the microwave switch is switched to the state where the microwave signal of the circulator 10 is allowed to pass through. The digital signal processor 3 performs FFT calculation and analysis on the sampling digital sequence, and calculates the real material level. After the signal is displayed, the display keyboard communication interface unit 1 uses a display method, or outputs to an external display instrument through a current, or communicates with an external display instrument through a communication interface, and transmits data.

Claims (1)

1. non-linear evolutional correction method for levelmeter of linear frequency-modulation radar, utilize the evolution principle to proofread and correct the frequency modulation of voltage controlled oscillator is non-linear, it is characterized in that: I. adopts and a kind ofly new to carry out closed-loop corrected principle based on evolvement method to pressuring controlling oscillator frequency is non-linear; II. utilize non-linear the non-linear of voltage controlled oscillator that remedy of fm voltage curve, the fm voltage curve is divided into plurality of sections, every section end points coordinate is divided certain region of variation, adopt the evolution principle to ask for the end points coordinate of corresponding best fm linearity, each end points connection can be formed the sweep voltage curve; III. adopt based on the method for difference frequency signal spectrum sigtral response and proofread and correct, need not measure the frequency or the phase place of high-frequency microwave signal, existing radar levelmeter does not need to increase any new hardware device can be proofreaied and correct, and perhaps increases a delayer at microwave unit and can realize online self-correcting; IV. can adopt the method for overlapping analysis to obtain the higher correction linearity.

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