A fast distance measurement method based on polarization modulation

Abstract

The present invention relates to the field of optical detection technology, and in particular to a polarization-modulated rapid distance measurement method, including S1, selecting a modulation frequency range of Z; S2, within the frequency range Z, gradually increasing the modulation frequency from the initial point of the frequency sweep with a frequency sweep step, comparing the voltage difference corresponding to the frequency sweep point with a threshold value, and then performing a reverse frequency sweep, outputting a first in-phase frequency according to the voltage difference corresponding to the frequency sweep point after the reverse frequency sweep with the threshold value; S3, shifting the initial point of the frequency sweep to the right by a frequency sweep curve cycle, and outputting a second in-phase frequency; S4, obtaining an accurate distance to be measured. The present invention avoids directly extracting a minimum point or searching for a minimum point in a small range at an extremely low signal-to-noise ratio, and indirectly obtains the position of the minimum point by measuring a point at a high signal-to-noise ratio; adopts variable step frequency sweeping to obtain an initial value of the in-phase frequency, thereby reducing the number of frequency sweeping points; and takes into account both measurement speed and accuracy, which is of great significance to improving the performance of the entire distance measurement system.

Description

Technical Field

[0001] The invention relates to the technical field of optical detection, and in particular to a polarization-modulated rapid distance measurement method. Background Art

[0002] With the rapid development of science and technology, the accuracy and speed requirements of laser ranging technology are becoming higher and higher in the manufacturing and assembly of large-scale equipment and scientific instruments, earth and atmosphere detection, aerospace and other fields. Polarization modulation ranging technology is based on phase laser ranging technology. It is based on the electro-optical modulator. The optical signal is modulated twice on the modulator. The phase difference between the two modulated signals on the modulator is demodulated by the polarization beam splitter and received by the photodetector. Different modulation frequencies correspond to different wavelengths of modulation signals. By sweeping the frequency, the output voltage of the detector at different frequencies is collected to obtain the modulation frequency-voltage curve. The modulation frequency corresponding to the two adjacent voltage minimum points on the sweeping frequency curve is measured, that is, the in-phase frequency, and then the distance is calculated. This technology is different from the traditional phase ranging method. It does not need to measure the phase, so it has obvious advantages in terms of measurement principle and can effectively improve the ranging accuracy.

[0003] Polarization modulation ranging technology converts the measurement of distance into the extraction of in-phase frequency. Therefore, the search speed and extraction accuracy of the in-phase frequency affect the performance of the entire ranging system. The traditional method of extracting the in-phase frequency is to perform a small step sweep in the entire sweep range, download the frequency-voltage offline and perform fitting to solve the minimum value. The sweep speed is slow and the efficiency is low. In recent years, some scholars have also differentiated the sweep curve and indirectly obtained the in-phase frequency by finding two adjacent zero points on the curve with negative left and positive right. Studying a suitable sweep strategy and improving the sweep speed and frequency extraction accuracy have great practical significance for improving the performance of the ranging system and its practicality. Summary of the invention

[0004] The technical problem to be solved by the present invention is to solve the problem of slow measurement speed in polarization modulation ranging and avoid extracting the in-phase frequency at a very low signal-to-noise ratio, thereby improving the measurement speed and measurement accuracy of the ranging system.

[0005] The technical solution adopted by the present invention is: a polarization modulation fast ranging method comprises the following steps:

[0006] S1. Select the modulation frequency range as Z, and perform frequency sweep according to the frequency sweep curve period;

[0007] Furthermore, the distance between the modulator and the reflector is measured and defined as the rough target value D. Since the distance between the modulator and the reflector is not easy to measure, a rough measurement module is added to the right side of the modulator. The distance between the rough measurement module and the reflector is approximated as the rough target value D. By the formula Calculate fb , where c is the speed of light and f b is the frequency difference between two adjacent in-phase frequencies; since the period T of the swept-frequency curve is approximately T≈f b , the period T of the swept-frequency curve is calculated;

[0008] S2. Within the frequency range Z, starting from the initial swept-frequency point, gradually increase the modulation frequency in steps of the swept-frequency step. Compare the voltage difference corresponding to the swept-frequency points with the first threshold and then perform reverse swept-frequency. According to the comparison between the voltage difference corresponding to the swept-frequency points after reverse swept-frequency and the second threshold, output the first in-phase frequency;

[0009] Further, it includes the following steps:

[0010] S21. Within the frequency range Z, starting from the swept-frequency starting point f0, with k as the swept-frequency step, gradually increase the modulation frequency, i.e., perform forward swept-frequency;

[0011] S22. Synchronously collect and store the sequence pairs of voltage values V i at different frequencies f i , where i = 0, 1, 2...; when i≥2, start to calculate the difference ΔV between the voltage value V i corresponding to the swept frequency f i and the voltage value V i-2 corresponding to the swept frequency f i-2 ;

[0012] S23. When the difference ΔV is greater than zero, stop the forward swept-frequency and record the corresponding frequency f Δ ;

[0013] S24. Select the frequency f Δ as the swept-frequency starting point and f Δ-2 as the swept-frequency ending point, start reverse swept-frequency, and modify the swept-frequency step to k / 4. Repeat steps S22 - S23. When ΔV is less than the set threshold m1, stop swept-frequency to obtain the initial value f'0 of the in-phase frequency;

[0014] S25. Set the modulation frequency to f'0 - h and f'0 + h, where h = T / 4, and collect the voltage values at the two frequencies respectively; take the difference ΔV2 between the voltage values corresponding to the modulation frequencies f'0 - h and f'0 + h;

[0015] S26. Given a threshold m2, compare ΔV2 with the threshold m2;

[0016] S27. If the absolute value of the voltage amplitude difference ΔV2 is less than the threshold m2, output the first in-phase frequency F1 and end the loop; otherwise, further judge the positive and negative of the voltage amplitude difference;

[0017] The value of the first in-phase frequency F1 is the modulation frequency corresponding to |Δv2| < m2;

[0018] S28. If the absolute value of the voltage difference in S27 is greater than the threshold value m2, further determine whether it is positive or negative. If it is positive, take the modulation frequency f'0 as the starting point, shift left by a step length s, and set the step length s to a constant that is one order of magnitude smaller than the threshold value m2, and go to S25. Otherwise, take the modulation frequency f'0 as the starting point, shift right by a step length s, and go to S25.

[0019] S3, shift the frequency sweep initial point in S2 to the right by a frequency sweep curve period T, and repeat S21-S28 to obtain a second in-phase frequency F2;

[0020] S4, calculating two adjacent in-phase frequencies f' according to the first in-phase frequency F1 and the second in-phase frequency F2 b , f' b =F2-F1, using the formula Find the precise distance D' to be measured; where n is the refractive index of air, c is the speed of light, F1 is the first in-phase frequency, N is an integer multiple of the modulation wavelength.

[0021] The beneficial effects of the present invention are:

[0022] 1. The method of the present invention avoids directly extracting the minimum point at a very low signal-to-noise ratio or searching for the minimum point in a small range, and indirectly obtains the position of the minimum point by measuring the point at a high signal-to-noise ratio;

[0023] 2. The method of the present invention studies a frequency sweeping strategy, adopts variable step frequency sweeping to obtain a roughly minimum point, and reduces the number of frequency sweeping points;

[0024] 3. The method of the present invention takes into account both measurement speed and accuracy, which is of great significance to improving the performance of the entire ranging system. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A flowchart of a polarization-modulated rapid ranging method of the present invention;

[0026] Figure 2 It is a structural schematic diagram of the polarization modulation ranging device adopted by the present invention;

[0027] Figure 3 This is a voltage signal diagram collected by the collection device in an embodiment of the present invention;

[0028] Figure 4 Schematic diagram of variable step frequency sweep in an embodiment of the present invention;

[0029] Figure 5 A schematic diagram of finding the voltage values ​​corresponding to points at equal frequency intervals h on both sides of the initial value of the in-phase frequency in an embodiment of the present invention;

[0030] Figure 2 In it, 1, laser, 2, polarization beam splitter, 3, modulator, 4, quarter wave plate, 5, reflector, 6, photodetector, 7, acquisition device, 8, microcontroller, 9, microwave source, 10, computer, 11, coarse measurement module. DETAILED DESCRIPTION

[0031] The present invention is further described below in conjunction with the accompanying drawings and embodiments. This figure is a simplified schematic diagram, which only illustrates the basic structure of the present invention in a schematic manner, and therefore it only shows the components related to the present invention.

[0032] like Figure 2 The figure shows the polarization modulation ranging system of the present invention. The working principle of the system is as follows: the laser output by the laser is polarized by the polarization beam splitter to generate a linear polarization light modulator at an angle of 45° to the X-axis, which modulates a microwave signal on the Y-axis component of the linear polarization light. The microwave signal is controlled by a microcontroller to generate a high-frequency sinusoidal signal with a continuously changing frequency under the control of a microwave source. After the modulated light is returned through a reflector, the phase information of the modulated signal will change with the distance. The reflected light enters the modulator again for secondary modulation. The secondary modulated light interferes in the polarization beam splitter, and the photodetector converts the optical signal into an electrical signal. The AD sampling and reception is controlled by the microcontroller, and the data is processed by the microcontroller and sent to the computer.

[0033] like Figure 1 As shown, a polarization modulation fast ranging method comprises the following steps:

[0034] S1. Select the modulation frequency range as 294.61MHz-362.58MHz, and perform frequency sweep according to the frequency sweep curve period;

[0035] The distance between the modulator and the reflector is measured and defined as the rough target value D. Since the distance between the modulator and the reflector is not easy to measure, a rough measurement module is added to the right side of the modulator. The distance between the rough measurement module and the reflector is measured to approximately obtain the rough target value of 3.8m. Calculate f b , where c is the speed of light, f b is the frequency difference between two adjacent in-phase frequencies; since the sweep frequency curve period T≈f b , calculate the sweep frequency curve period; T = 39.47MHz;

[0036] S2, within the frequency range Z, gradually increase the modulation frequency from the initial point of the frequency sweep with a frequency sweep step, compare the voltage difference corresponding to the frequency sweep point with the first threshold, and then perform a reverse frequency sweep, and output a first in-phase frequency according to the voltage difference corresponding to the frequency sweep point after the reverse frequency sweep with the second threshold;

[0037] like Figure 3As shown, S21, within the frequency range Z, starting from the frequency sweep starting point f0 = 294.61 MHz, with k = 433.48 KHz as the frequency sweep step, gradually increasing the modulation frequency, i.e., forward sweep;

[0038] S22, synchronously collect voltage values ​​at different frequencies, and store them in order of frequency. i and voltage sequence V i , where i=0, 1, 2…; when i≥2, the sweep frequency starts to be calculated as f i Corresponding voltage value V i With the sweep frequency f i-2 The corresponding voltage value V i-2 The difference ΔV;

[0039] like Figure 4 As shown, S23, when the difference ΔV is greater than zero, stop the forward sweep and record the corresponding frequency f at this time Δ ;

[0040] S24, select the frequency range (f Δ-2 , f Δ ), start reverse sweep, and modify the sweep step to k / 4, repeat steps S22-S23, when ΔV is less than the set threshold m1=1.5*10 -4 , stop frequency sweeping; get the initial value of the in-phase frequency f'0=307.70MHz;

[0041] like Figure 5 As shown, S25, set the modulation frequency to f'0-h and f'0+h, where h = T / 4, and collect the voltage values ​​v at the two frequencies respectively. a and v b ; The voltage values ​​corresponding to the modulation frequencies f'0-h and f'0+h are calculated as the difference ΔV2;

[0042] S26. Give a threshold m2=1*10 4 , compare ΔV2 with the threshold m2;

[0043] S27, if the absolute value of the voltage amplitude difference is less than the threshold m2, then output the first in-phase frequency F1 = 307.79 MHz, and end the algorithm; otherwise, further determine whether the voltage amplitude difference is positive or negative;

[0044] S28. If the absolute value of the voltage difference in S27 is greater than the threshold value m2, further determine whether it is positive or negative. If it is positive, take the modulation frequency f'0 as the starting point, shift left by a step length s=1KHz, and set the value of the step length s to a constant that is one order of magnitude smaller than the threshold value m2, and go to S25. Otherwise, take the modulation frequency f'0 as the starting point, shift right by a step length s, and go to S25.

[0045] S3, shift the initial point of the frequency sweep to the right by a frequency sweep curve period T, and repeat S21-S28 to obtain the second in-phase frequency F2=346.29MHz;

[0046] S4, calculating two adjacent in-phase frequencies f' according to the first in-phase frequency F1 and the second in-phase frequency F2 b , f' b =F2-F1, using the formula Find the precise distance to be measured D'=3.8988m; where n is the refractive index of air, c is the speed of light, F1 is the first in-phase frequency, N is an integer multiple of the modulation wavelength.

[0047] The method of the present invention adopts online measurement technology, and has a fast measurement speed; it avoids directly extracting a minimum point at an extremely low signal-to-noise ratio or searching for a minimum point in a small range, and indirectly obtains the position of the minimum point by measuring a point at a high signal-to-noise ratio; it adopts variable step frequency sweeping to obtain an initial value of the in-phase frequency, and reduces the number of frequency sweeping points; it takes into account both the measurement speed and the accuracy, and is of great significance to improving the performance of the entire distance measurement system.

[0048] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A polarization-modulated rapid ranging method, characterized in that: The following steps are involved: S1. Select the modulation frequency range as Z, and perform frequency sweep according to the frequency sweep curve period; The frequency sweep curve period is obtained by measuring the coarse target value D between the coarse measurement module and the reflector, according to the formula Calculate f b , where c is the speed of light, f b is the frequency difference between two adjacent in-phase frequencies; since the sweep frequency curve period T≈f b , calculate and obtain the period T of the frequency sweep curve; S2, within the frequency range Z, gradually increase the modulation frequency from the initial point of the frequency sweep with a frequency sweep step, compare the voltage difference corresponding to the frequency sweep point with the first threshold, and then perform a reverse frequency sweep, and output a first in-phase frequency according to the voltage difference corresponding to the frequency sweep point after the reverse frequency sweep with the second threshold; The S2 comprises the following steps: S21, within the frequency range Z, starting from the frequency sweep starting point f0, with k as the frequency sweep step, gradually increasing the modulation frequency, i.e., forward frequency sweep; S22, synchronously collect voltage values ​​at different frequencies, that is, store frequency f i and voltage value V i sequence pair, where i = 0, 1, 2...; when i ≥ 2, the sweep frequency starts to be calculated as f i Corresponding voltage value V i With the sweep frequency f i-2 The corresponding voltage value V i-2 The difference ΔV; S23, when the difference ΔV is greater than zero, stop the forward sweep and record the corresponding frequency f at this time Δ ; S24, select frequency f Δ is the frequency sweep starting point, f Δ-2 The frequency sweeping end point is reached, and the frequency sweeping step is modified to k / 4. Steps S22-S23 are repeated. When ΔV is less than the set threshold m1, the frequency sweeping is stopped. The initial value of the in-phase frequency f'0 is obtained. S25, setting the modulation frequency to f'0-h and f'0+h, where h=T / 4, respectively collecting the voltage values ​​at the two frequencies; taking the difference ΔV2 between the voltage values ​​corresponding to the modulation frequencies f'0-h and f'0+h; S26, given a threshold m2, comparing ΔV2 with the threshold m2; S27, if the absolute value of the voltage amplitude difference ΔV2 is less than the threshold m2, the first in-phase frequency F1 is output to end the cycle; otherwise, the positive or negative value of the voltage amplitude difference is further determined; S28, if the absolute value of the voltage difference in S27 is greater than the threshold m2, further determine whether it is positive or negative. If it is positive, take the modulation frequency f'0 as the starting point, shift left by a step length s, and set the size of the step length s to be a constant that is one order of magnitude smaller than the threshold m2, and go to step S25; otherwise, take the modulation frequency f'0 as the starting point, shift right by a step length s, and go to step S25; S3, shift the initial point of the frequency sweep to the right by one frequency sweep curve period, and repeat step S2 to output a second in-phase frequency; S4, calculating the precise distance to be measured according to the first in-phase frequency and the second in-phase frequency; The method of obtaining an accurate distance to be measured according to the first in-phase frequency and the second in-phase frequency comprises: Calculate two adjacent in-phase frequencies f' b , f' b =F2-F1, where F1 is the first in-phase frequency and F2 is the second in-phase frequency; Using the formula Find the precise distance D' to be measured; where n is the refractive index of air, c is the speed of light, F1 is the first in-phase frequency, N is an integer multiple of the modulation wavelength.

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