Frequency modulation laser interference ranging nonlinear phase error compensation system and method

Abstract

The invention relates to the technical field of frequency modulation laser interference ranging, in particular to a frequency modulation laser interference ranging nonlinear phase error compensation system and method, which can realize effective compensation of intermediate frequency signal phase errors and improve ranging precision under the condition that the algorithm complexity is not greatly improved. According to the system, the nonlinear phase error of the intermediate frequency signals of the reference branch and the measurement branch is accurately compensated, so that the phase error caused by frequency modulation nonlinearity is remarkably reduced, and the overall ranging precision of the laser interference ranging system is further improved. Intermediate frequency signal nonlinear phase error compensation is adopted, the nonlinear phase error is effectively compensated under the condition that hardware consumption is not greatly improved, and finally the ranging precision of the system is improved.

Description

Technical Field

[0001] The invention relates to the technical field of frequency-modulated laser interferometry ranging, and in particular to a frequency-modulated laser interferometry ranging nonlinear phase error compensation system and method. Background Art

[0002] Frequency-modulated laser interferometry ranging technology is a high-precision absolute ranging technology for medium and short distances. It has the advantages of non-cooperation and non-contact. It has broad application prospects in high-precision large-size three-dimensional morphology measurement fields such as rocket assembly error measurement and wind turbine blade surface measurement.

[0003] In the frequency modulated continuous wave laser ranging system, the nonlinearity of the laser emission signal frequency cannot be completely eliminated by nonlinear pre-correction. Therefore, there will be nonlinear phase errors caused by nonlinear frequency modulation in the intermediate frequency signal. The nonlinear phase error is reflected in the spectrum of the signal as a widening of the frequency range centered on the actual frequency, and it is impossible to obtain an ideal spectrum with a single peak value, resulting in poor frequency measurement accuracy, thereby reducing the distance measurement accuracy. Therefore, in order to improve the accuracy of the system distance measurement, the nonlinear phase error caused by nonlinear frequency modulation must be compensated.

[0004] However, there is no corresponding technology for compensating the nonlinear phase error of the frequency modulated continuous wave laser ranging system. At present, the equal-frequency sampling method is the main method to suppress nonlinearity. The principle of the equal-frequency sampling method is to use the signal of the reference optical path as a clock to sample the interference signal of the measurement optical path, compensate for the frequency sweep linearity of the tunable laser, and then eliminate the influence of frequency modulation nonlinearity. Although theoretically equal-frequency sampling can eliminate frequency modulation nonlinearity, the data obtained after equal-frequency sampling is not equidistant in the time domain. When processing data, the measurement data must be subjected to non-equidistant time interval Fourier transform to extract the frequency of the measurement signal. However, the non-equidistant time interval Fourier transform algorithm is complex and immature, and the data processing volume is large, so it is difficult to use the underlying hardware to achieve fast real-time solution. Summary of the invention

[0005] In view of this, the present invention provides a frequency modulated laser interferometer ranging nonlinear phase error compensation system and method, which can effectively compensate for the phase error of the intermediate frequency signal without significantly increasing the algorithm complexity, thereby improving the ranging accuracy.

[0006] To achieve the above purpose, the technical solution of the present invention is as follows:

[0007] A frequency-modulated laser interferometric ranging nonlinear phase error compensation system comprises an interferometric optical path with a fixed delay optical path as a reference branch, a measuring branch, a processing unit and a laser light source; the laser light source generates a laser signal with a linear optical frequency change after nonlinear pre-correction, and the laser signal enters the reference branch and the measuring branch respectively; the reference branch and the measuring branch generate intermediate frequency signals respectively, and the frequency of the intermediate frequency signal is related to the distance; photoelectric detectors respectively located on the reference branch and the measuring branch are used to receive the corresponding intermediate frequency signals, and two ADCs in the circuit system are used to sample and digitize the intermediate frequency signals output by the photoelectric detectors; the processing unit is used to compensate the phase error of the intermediate frequency signal of the measuring branch by using the phase error of the intermediate frequency signal of the reference branch.

[0008] Among them, the processing unit calculates the change of the phase of the reference branch intermediate frequency signal over time, and subtracts it from the phase distribution of the ideal reference branch intermediate frequency signal to obtain the phase error distribution of the reference branch intermediate frequency signal; amplifies the phase error of the reference branch intermediate frequency signal to obtain the compensation value of the phase error of the measuring branch intermediate frequency signal; and uses the compensation value to correct the phase of the measuring branch intermediate frequency signal.

[0009] Among them, it also includes a unit for amplifying the phase error of the intermediate frequency signal of the reference branch, and the specific amplification implementation process is:

[0010] Perform a rough frequency measurement on the intermediate frequency signal of the measurement branch to obtain a rough measured frequency;

[0011] Divide the rough measured frequency by the ideal frequency of the reference branch intermediate frequency signal to obtain the amplification factor of the phase error;

[0012] The phase error distribution of the intermediate frequency signal of the reference branch is multiplied by the amplification factor to obtain the phase error compensation amount distribution of the intermediate frequency signal of the measurement branch.

[0013] The system also includes: a digital filtering module for preprocessing the collected digital signal; a spectrum analysis module for performing spectrum analysis on the intermediate frequency signals of the reference branch and the measurement branch, and calculating their rough frequency values ​​respectively; a Hilbert transform module for performing Hilbert transform on the intermediate frequency signals of the reference branch and the measurement branch, and constructing the corresponding analytical function;

[0014] CORDIC-arctan operation module, used to extract the phase of the analytical function.

[0015] The present invention also provides a method for compensating nonlinear phase error of frequency modulated laser interferometer ranging, which is implemented by the system of the present invention and includes the following steps:

[0016] Obtaining the phase error distribution of the reference branch intermediate frequency signal in the time domain;

[0017] Amplifying the phase error of the intermediate frequency signal of the reference branch as a compensation value for the phase error of the intermediate frequency signal of the measurement branch;

[0018] The compensation value is used to compensate for the phase error of the intermediate frequency signal of the measurement branch.

[0019] It also includes:

[0020] Perform Hilbert transform on the intermediate frequency signal of the measurement branch and construct the analytical function of the measurement branch;

[0021] Take a negative value for the phase error compensation value of the intermediate frequency signal of the measurement branch, and perform sin and cos operations to obtain its analytical function;

[0022] The analytical function of the intermediate frequency signal of the measuring branch is complex multiplied by the analytical function of the phase error compensation value to obtain the compensated intermediate frequency signal of the measuring branch.

[0023] Beneficial effects:

[0024] 1. The system of the present invention significantly reduces the phase error caused by frequency modulation nonlinearity by accurately compensating the nonlinear phase error of the intermediate frequency signal of the reference branch and the measurement branch, thereby improving the overall ranging accuracy of the laser interferometer ranging system. The intermediate frequency signal nonlinear phase error compensation is adopted to effectively compensate for the nonlinear phase error without much increase in hardware consumption, and finally improve the ranging accuracy of the system.

[0025] 2. The system of the present invention adopts a dual interference optical path design and combines a high-speed ADC to digitally process the output signal of the photodetector, so that the system can maintain a good working state under various environmental conditions, thereby improving the stability and reliability of the system.

[0026] 3. The present invention utilizes the phase error of the reference branch intermediate frequency signal to compensate for the phase error of the measurement branch intermediate frequency signal, thereby avoiding complex mathematical modeling and calculation, simplifying the process of nonlinear phase error compensation, and reducing the complexity of system implementation.

[0027] 4. The present invention is not only suitable for static target ranging, but also suitable for dynamic target ranging occasions, especially in application scenarios requiring high precision and fast response, such as precision manufacturing, aerospace and other fields, and has broad application prospects; the present invention improves data processing efficiency and accelerates ranging speed by adopting advanced digital signal processing technologies such as digital filtering, Hilbert transform, CORDIC algorithm, etc., while ensuring the accuracy of data processing.

[0028] 5. The method of the present invention is implemented based on the system of the present invention. By accurately compensating for the nonlinear phase error of the intermediate frequency signals of the reference branch and the measurement branch, the phase error caused by the nonlinearity of the frequency modulation is significantly reduced, thereby improving the overall ranging accuracy of the laser interferometer ranging system.

[0029] 6. The method of the present invention adopts a dual interference optical path design and combines a high-speed ADC to digitally process the output signal of the photodetector, so that the system can maintain a good working state under various environmental conditions, thereby improving the stability and reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the phase error compensation process of the system of the present invention.

[0031] Figure 2 This is a hardware implementation block diagram of the phase error compensation of the system of the present invention.

[0032] Figure 3 Schematic diagram of frequency change before phase error compensation in an embodiment of the method of the present invention.

[0033] Figure 4 Schematic diagram of frequency change after phase error compensation in an embodiment of the method of the present invention. DETAILED DESCRIPTION

[0034] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0035] Aiming at the problem that the intermediate frequency signal in the frequency modulation laser interferometer ranging system has nonlinear phase error caused by frequency modulation nonlinearity and cannot accurately calculate the frequency of the intermediate frequency signal, the present invention provides a frequency modulation laser interferometer ranging nonlinear phase error compensation system, comprising two paths, an interference optical path with a fixed delay optical path as a reference branch, and another path as a measurement branch; after the laser light source is pre-corrected nonlinearly, the laser signal with linear optical frequency change enters the reference branch and the measurement branch respectively; the reference branch and the measurement branch respectively interfere to generate an intermediate frequency signal, and the frequency of the intermediate frequency signal is related to the distance; the photoelectric detectors of the reference branch and the measurement branch respectively receive the intermediate frequency signal of the reference branch and the intermediate frequency signal of the measurement branch, and the two high-speed ADCs in the circuit system respectively sample the output signals of the photoelectric detectors to digitize the two intermediate frequency signals; the phase error of the intermediate frequency signal of the reference branch in the ranging system is used to compensate the phase error of the intermediate frequency signal of the measurement branch. Since the signals in the reference branch and the measurement branch are homologous, they contain the same frequency modulation nonlinear information, and the only difference is that the optical paths of the two branches are different, so the nonlinear phase errors of the two branches are connected. Under the premise that the phase error belongs to a stationary random process, the nonlinear phase errors of the two branches are only related to their respective optical paths. The phase error of the reference branch is a calculable quantity. By multiplying it with the optical path multiple between the measurement branch and the reference branch, an estimated value of the phase error of the measurement branch can be obtained. Therefore, the phase error of the measurement branch can be compensated. The phase error compensation process of the system of the present invention is as follows: Figure 1 The hardware implementation block diagram of phase error compensation is shown in Figure 2 shown.

[0036] The present invention also proposes a method for compensating nonlinear phase error of frequency modulated laser interferometer ranging, which is implemented based on the system of the present invention and includes the following steps:

[0037] 1) Obtain the phase error distribution of the reference branch intermediate frequency signal in the time domain. Calculate the distribution of the reference branch intermediate frequency signal phase over time The phase error distribution of the reference branch intermediate frequency signal is obtained by subtracting it from the phase distribution of the ideal reference branch intermediate frequency signal.

[0038]

[0039] Among them, the phase distribution of the ideal reference signal is:

[0040]

[0041] In the above formula, Rr is the optical path of the laser in the reference branch, and its value is the length of the delay fiber multiplied by the fiber refractive index.

[0042] 2) Amplify the phase error of the reference branch intermediate frequency signal. Perform a rough frequency measurement on the filtered measurement branch intermediate frequency signal, divide the rough frequency obtained by the ideal frequency of the reference branch intermediate frequency signal to obtain the amplification factor n of the reference branch intermediate frequency signal phase error, and multiply the reference branch intermediate frequency signal phase error distribution by n to obtain the measurement branch intermediate frequency signal phase error compensation amount distribution:

[0043]

[0044] 3) Compensate for the phase error of the intermediate frequency signal in the measurement branch. The phase of the intermediate frequency signal S(t) of the measurement branch is compensated to obtain the intermediate frequency signal of the measurement branch after nonlinear phase error compensation:

[0045]

[0046] Through the above process, the nonlinear phase error of the intermediate frequency signal of the measurement branch can be approximately compensated.

[0047] Specifically, the method of the present invention is implemented in the following steps:

[0048] (1) The first step is to build a dual-interference optical path frequency-modulated continuous wave laser ranging system, in which the interference optical path with a fixed delay optical path is used as the reference path, and the other path is used as the measurement path. After the laser light source is pre-corrected nonlinearly, the laser signal with linear optical frequency changes enters the reference branch and the measurement branch respectively. The reference path and the measurement path interfere with each other to generate an intermediate frequency signal, and the frequency of the intermediate frequency signal is related to the distance.

[0049] (2) Step 2: The photodetectors of the reference branch and the measurement branch receive the intermediate frequency signal of the reference branch and the intermediate frequency signal of the measurement branch respectively. Two high-speed ADCs in the circuit system sample the output signals of the photodetectors respectively and digitize the two intermediate frequency signals.

[0050] (3) The third step: perform preprocessing such as digital filtering on the collected digital signal.

[0051] (4) Step 4: Perform spectrum analysis on the reference branch intermediate frequency signal and the measurement branch intermediate frequency signal, calculate their rough frequency values ​​fr and fm, and calculate n = fm / fr as the phase error amplification factor.

[0052] (5) Step 5: Perform Hilbert transform on the reference branch intermediate frequency signal. Assume that the reference branch intermediate frequency signal before the transform is r r (t), the transformed reference branch intermediate frequency signal is r i (t), then R(t)=r r (t)+j×r i(t) is the analytical function of the measured signal. The phase distribution of the reference branch intermediate frequency signal is obtained by performing CORDIC-arctan operation on the analytical function.

[0053] (6) Step 6: The phase difference between the ideal reference branch intermediate frequency signal and the reference branch intermediate frequency signal is used to obtain the phase error of the reference branch intermediate frequency signal. Since the delay arm length of the reference branch is a fixed value, the accurate value of the reference branch intermediate frequency signal can be obtained when performing nonlinear pre-correction, which will not be described here. The phase distribution of the ideal reference branch intermediate frequency signal is Phase Error

[0054] (7) Step 7: Amplify the phase error of the intermediate frequency signal of the reference branch as the compensation value of the phase error of the intermediate frequency signal of the measurement branch:

[0055] (8) Step 8: Perform Hilbert transform on the intermediate frequency signal of the measurement branch to construct the analytical function of the measurement branch for subsequent phase error compensation. The construction method of the analytical function of the measurement branch is the same as that of the fifth step. The analytical function of the intermediate frequency signal of the measurement branch is finally obtained as M(t)=m r (t)+j×m i (t).

[0056] (9) Step 9: First, measure the phase error compensation value of the intermediate frequency signal of the branch Taking the negative value, we get Then perform sin and cos operations to obtain its analytical function: Δφ(t) is used for subsequent phase error compensation of the intermediate frequency signal of the measurement branch.

[0057] (10) Step 10: Perform complex multiplication on the analytical function M(t) of the intermediate frequency signal of the measurement branch and Δφ(t) to obtain the compensated intermediate frequency signal of the measurement branch: M correct (t) = M(t) × Δφ(t).

[0058] The effect of the method proposed in the present invention is verified using the actual collected measurement data. Figure 3 It is the frequency distribution obtained by performing 1000 consecutive measurements on the echo intermediate frequency signal at a distance of 9m before phase error compensation. The frequency measurement range at this time is 256Hz. Figure 4 This is the frequency distribution calculated after phase error compensation for the same set of data. At this time, the range of frequency measurement is 77Hz, which is more than 3 times smaller than before compensation. It can be seen that after phase error compensation, the repeatability of frequency measurement has been greatly improved.

[0059] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A frequency modulated laser interferometer ranging nonlinear phase error compensation system, characterized in that: The invention comprises an interference optical path with a fixed delay optical path as a reference branch, a measurement branch, a processing unit and a laser light source; the laser light source generates a laser signal with a linear change in optical frequency after nonlinear pre-correction, and the laser signal enters the reference branch and the measurement branch respectively; the reference branch and the measurement branch generate intermediate frequency signals respectively, and the frequency of the intermediate frequency signal is related to the distance; The photoelectric detectors located on the reference branch and the measuring branch are used to receive the corresponding intermediate frequency signals. The two ADCs in the circuit system are used to sample and digitize the intermediate frequency signals output by the photoelectric detectors. The processing unit is used to compensate the phase error of the intermediate frequency signal of the measuring branch by using the phase error of the intermediate frequency signal of the reference branch.

2. The system according to claim 1, characterized in that The processing unit calculates the change of the phase of the reference branch intermediate frequency signal over time, and subtracts it from the phase distribution of the ideal reference branch intermediate frequency signal to obtain the phase error distribution of the reference branch intermediate frequency signal; amplifies the phase error of the reference branch intermediate frequency signal to obtain a compensation value for the phase error of the measuring branch intermediate frequency signal; and uses the compensation value to correct the phase of the measuring branch intermediate frequency signal.

3. The system according to claim 1 or 2, characterized in that , also includes a unit for amplifying the phase error of the intermediate frequency signal of the reference branch, and the specific amplification implementation process is: Perform a rough frequency measurement on the intermediate frequency signal of the measurement branch to obtain a rough measured frequency; The rough measurement frequency is divided by the ideal frequency of the reference branch intermediate frequency signal to obtain the phase error amplification factor; the phase error distribution of the reference branch intermediate frequency signal is multiplied by the amplification factor to obtain the phase error compensation amount distribution of the measurement branch intermediate frequency signal.

4. The system according to claim 3, characterized in that , also includes: a digital filtering module, which is used to pre-process the collected digital signal; a spectrum analysis module, which is used to perform spectrum analysis on the intermediate frequency signals of the reference branch and the measurement branch, and calculate their rough frequency values ​​respectively; a Hilbert transform module, which is used to perform Hilbert transform on the intermediate frequency signals of the reference branch and the measurement branch, and construct the corresponding analytical function; a CORDIC-arctan operation module, which is used to extract the phase of the analytical function.

5. A method for compensating nonlinear phase error in frequency modulated laser interferometry ranging, implemented by a system as claimed in any one of claims 1 to 5, characterized in that: The following steps are involved: Obtaining the phase error distribution of the reference branch intermediate frequency signal in the time domain; The phase error of the intermediate frequency signal of the reference branch is amplified as a compensation value for the phase error of the intermediate frequency signal of the measuring branch; and the phase error of the intermediate frequency signal of the measuring branch is compensated by using the compensation value.

6. The method according to claim 5, characterized in that Also includes: Perform Hilbert transform on the intermediate frequency signal of the measurement branch and construct the analytical function of the measurement branch; Take a negative value for the phase error compensation value of the intermediate frequency signal of the measurement branch, and perform sin and cos operations to obtain its analytical function; The analytical function of the intermediate frequency signal of the measuring branch is complex multiplied by the analytical function of the phase error compensation value to obtain the compensated intermediate frequency signal of the measuring branch.

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