A high-precision distance measurement method for unmanned aerial vehicle inspection

Abstract

The present application belongs to the field of automatic inspection technology for distribution networks, and in particular, relates to a high-precision distance measurement method for unmanned aerial vehicle inspection. The method includes the following steps: configuring a laser distance measurement and correction system; establishing a distance measurement and correction system operation model; decomposing and simplifying the operation model; and correcting the simplified operation model. The high-precision distance measurement method for unmanned aerial vehicle inspection of the present application utilizes secondary sampling data and the phase relationship of dual-path interference signals, and corrects the initial sampling error and the influence of system delay through correction signals, thereby improving the accuracy of the measurement method and achieving the purpose of improving the measurement accuracy.

Description

Technical Field

[0001] The present application belongs to the technical field of automatic inspection of distribution networks, and in particular, relates to a high-precision ranging method for unmanned aerial vehicle inspection. Background Art

[0002] With the extensive application of drone inspection technology in power grid maintenance and monitoring operations, the efficiency of various basic operation tasks has been effectively improved based on drone images and data from various sensors. Among them, optical frequency modulated laser ranging is a non-contact ranging method used for drones. Because drones generally have low flight altitudes and flight speeds, and the straight-line distance between drones and the measured object is generally within 100 meters, it belongs to close-range ranging, and the measured distance is much smaller than the speed of light. Therefore, this method can quickly and efficiently obtain the distance between the target and the drone during the drone inspection flight to achieve rapid ranging and positioning. Optical frequency modulated laser ranging is a method for measuring the distance of coherent detection targets. This measurement method requires the use of a modulated light source to achieve accurate measurement by controlling and changing the bandwidth of the light source. In the traditional optical frequency modulated laser ranging system, the frequency of the light source is related to the distance to be measured, but in the actual modulation process, due to the influence of factors such as equipment structure and accuracy, the modulation process does not change linearly, thus affecting the measurement accuracy and ranging range of this method. Summary of the invention

[0003] The purpose of this application is to provide a high-precision ranging method for unmanned aerial vehicle inspection for improving the measurement accuracy of optical frequency modulated laser ranging and reducing the influence of uncontrollable factors in the optical frequency modulation process.

[0004] To achieve the above objectives, this application adopts the following technical solutions.

[0005] A high-precision distance measurement method for unmanned aerial vehicle inspection includes the following steps:

[0006] Step 1: Configure the laser ranging and calibration system

[0007] It includes a laser 1, a first beam splitter 2, a second beam splitter 3, a circulator 4, a coherent detector 5, a Mach-Zehnder interferometer 6, and a collector 8, which are arranged on the UAV;

[0008] The laser 1 is used to generate a modulated laser signal. The laser signal is split into a detection signal and an auxiliary signal by a first beam splitter 2. The detection signal is split into a measurement signal and an interference signal by a second beam splitter 3. The measurement signal is irradiated to the target to be measured by a circulator 4. The reflected light signal returns to the circulator 4 to obtain a reflected signal. The reflected signal is coherent with the interference signal and then collected by a coherent detector 5 to obtain a measurement signal. The auxiliary signal is passed through a Mach-Zehnder interferometer 6 to obtain a correction signal. The measurement signal and the correction signal are collected by a collector 8.

[0009] Step 2: Establish the distance measurement and correction system operation model

[0010] Based on the basic principles of optical frequency modulation and signal interference, we can obtain

[0011] The electric field expression of the detection signal: E 1 (t) = H 1 cos[2πf s t+πvt 2 +θ 0 (t)]

[0012] Electric field expression of reflected signal: E 2 (t) = H 2 cos[2πf s (t-δ 2 )+πv(t-δ 2 ) 2 +θ 0 (t)]

[0013] The measurement signal is the interference signal obtained by the coherence of the detection signal and the reflection signal. The correction signal is the interference signal obtained by the auxiliary signal after passing through the Mach-Zehnder interferometer 6. Based on the photoelectric square law, it can be obtained:

[0014] Current expression of the correction signal: I 3 (t) = H 3 cos[2πf(t)δ 3 ]

[0015] Current expression of the measurement signal: I 4 (t) = H 4 cos[2πf(t)δ 4 ]

[0016] In the formula, H 1 is the detection electric field amplitude; H 2 is the reflected electric field amplitude; is the frequency modulation speed, f e is the final laser modulation frequency, f s is the initial laser modulation frequency, T is the frequency modulation period; δ 2 (t) is the reflected signal delay; θ 0 is the initial phase; μ is the interference efficiency; H 3 is the correction signal amplitude; H 4 is the measured signal amplitude; δ 3 To correct the signal delay; δ 4 To measure signal delay;

[0017] Step 3: Decomposition and simplification of the computational model

[0018] The instantaneous speed of optical frequency modulation is fitted by a polynomial and simplified to obtain the electric field expression output by the laser:

[0019] In the formula, v i is the i-th coefficient of the modulation speed, i = 1, 2, 3; f(t) is the instantaneous optical frequency of the modulation;

[0020] Similarly, the electric field expression of the reflected signal and the current expression of the measured signal are obtained:

[0021]

[0022]

[0023] Considering that the distance between the object being measured and the drone is generally within 100 meters during the drone inspection process, which is much smaller than the speed of light, the delay in the corresponding space can be ignored, and the electric field expression of the measurement signal can be simplified to:

[0024]

[0025] where f(t) = f s +v 1 t+v 2 t 2 +v 3 t 3 is the sampling time;

[0026] Then I' 4 (t) = cos[2πδ 4 f(t)];

[0027] The phase of the measured signal can be expressed as: 4 (t)=2πf(t)δ 4 ;

[0028] Step 4: Correction based on the simplified computational model

[0029] Considering that the modulation speed is not constant, the electric field expression of the detection signal and the reflected signal can be transcribed as:

[0030] E 1 '(t)=H 1 cos[2πf s (t-δ 1 )+πv(t-δ 1 ) 2 +θ 0 (t)]

[0031] E' 2 (t) = E 1 '(t-δ 2 )

[0032] Based on the photoelectric square law, the corrected current expression of the measured signal is obtained after filtering the DC signal and the unmeasurable signal in the signal:

[0033] I 4 =H 4 cos[2πf s δ 4 +2πv(t)δ 4 t]=H 4 cos[2πδ 4 [f s +v(t)t]=H 4 cos[2πδ 4 f(t)];

[0034] Similarly, the corrected current expression of the correction signal can be obtained: I 3 =H 3 cos[2πδ 3 f(t)];

[0035] The measured signal is sampled twice using the time points of the correction signal peak and trough as the sampling moments. The frequency interval corresponding to adjacent sampling points is: The phase interval corresponding to adjacent sampling points is Δθ = π;

[0036] Then the secondary sampling time f(t) satisfies 2πδ 3 f(t)=π·N,N=1,2,3...;

[0037] After substituting, we get the corrected electrical quantity expression of the measured signal after secondary sampling:

[0038] The corrected measured distance expression based on the sub-sampling frequency information is obtained:

[0039] In a further improvement or perfection of the aforementioned high-precision ranging method for UAV inspection, the laser source refers to a light frequency modulated laser source.

[0040] In order to further improve or perfect the above-mentioned high-precision ranging method for UAV inspection, in the laser ranging and correction system, the power division ratio of the first beam splitter 2 and the second beam splitter 3 is 9:1.

[0041] To further improve or perfect the above-mentioned high-precision ranging method for UAV inspection, the laser ranging and correction system also includes a collimator 9 arranged on the rear side of the circulator 4.

[0042] Its beneficial effects are:

[0043] The high-precision ranging method for drone inspection in the present application utilizes secondary sampling data and the phase relationship of dual-path interference signals, corrects the initial sampling error and the influence of system delay through correction signals, improves the accuracy of the measurement method, and achieves the purpose of improving measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a schematic diagram of the structure of the laser ranging and correction system;

[0045] The reference numerals include:

[0046] Laser 1, first beam splitter 2, second beam splitter 3, circulator 4, coherent detector 5, Mach-Zehnder interferometer 6, collector 8, collimator 9. DETAILED DESCRIPTION

[0047] The present application is described in detail below in conjunction with specific embodiments.

[0048] The high-precision ranging method for drone inspection in the present application uses a spectrometer to construct an auxiliary correction signal based on the traditional optical frequency modulation ranging system, and corrects the original sampling data through secondary sampling, avoiding the problem that the comprehensive cost and measurement accuracy of traditional solutions such as electrical parameter control and cavity length coordination are difficult to meet the requirements.

[0049] The high-precision ranging method for drone inspection in this application includes the following specific steps:

[0050] Step 1: Configure the UAV inspection laser ranging and calibration system

[0051] It includes a laser 1, a first beam splitter 2, a second beam splitter 3, a circulator 4, a coherent detector 5, a Mach-Zehnder interferometer 6, and a collector 8, which are arranged on the UAV;

[0052] The laser 1 is used to generate a modulated laser signal. The laser signal is split into a detection signal and an auxiliary signal by a first beam splitter 2. The detection signal is split into a measurement signal and an interference signal by a second beam splitter 3. The measurement signal is irradiated to the target to be measured by a circulator 4. The reflected light signal returns to the circulator 4 to obtain a reflected signal. The reflected signal is coherent with the interference signal and then collected by a coherent detector 5 to obtain a measurement signal. The auxiliary signal is passed through a Mach-Zehnder interferometer 6 to obtain a correction signal. The measurement signal and the correction signal are collected by a collector 8.

[0053] The drone inspection laser ranging and correction system of the present application avoids the drawbacks of the original system by creating an auxiliary correction loop based on the traditional nonlinear optical frequency modulation correction structure.

[0054] Step 2: Establish the distance measurement and correction system operation model

[0055] Based on the UAV inspection laser ranging and correction system in step 1, the optical signal of the laser is split by the first spectrometer to generate a detection signal and an auxiliary signal, and the detection signal is divided into a ranging signal and an interference signal by the second spectrometer. The ranging signal is illuminated by the circulator to the target to be measured, and the circulator collects the reflected light to form a reflected signal. The reflected signal and the interference signal are coherent in the coherent detector to obtain a measurement signal, which is acquired by the collector. In the traditional scheme, the delay difference of the two coherent signals that generate the measurement signal is used to realize the indirect measurement of the distance, and the frequency modulation is used to realize the control of the measurement range. According to the basic principles of optical frequency modulation and signal interference, it can be obtained

[0056] The electric field expression of the detection signal: E 1 (t) = H 1 cos[2πf s t+πvt 2 +θ 0 (t)]

[0057] Electric field expression of reflected signal: E 2 (t) = H 2 cos[2πf s (t-δ 2 )+πv(t-δ 2 ) 2 +θ 0 (t)]

[0058] Similarly, the measurement signal is the interference signal obtained by the coherence of the detection signal and the reflected signal, and the correction signal is the interference signal obtained by the auxiliary signal after passing through the Mach-Zehnder interferometer 6. In the actual implementation process, since the signal modulation is not an ideal linear modulation process, it is necessary to use a polynomial to fit the instantaneous modulation rate to analyze the signal, and then the universal electric field expression of the modulated signal can be obtained.

[0059]

[0060] Combined with the square law characteristics of the photodetector, it can be seen that the output current satisfies I=RP, R is the responsivity, P is the power, and the output current is proportional to the amplitude of the incident signal. The current expression of the correction signal and the measurement signal can be obtained:

[0061] Current expression of the correction signal: I 3 (t) = H 3 cos[2πf(t)δ 3 ]

[0062] Current expression of the measurement signal: I 4 (t) = H 4 cos[2πf(t)δ 4 ]

[0063] Where f(t) = f s +v 1 t+v 2 t 2 +v 3 t 3 is the sampling time; H 1 is the detection electric field amplitude; H 2 is the reflected electric field amplitude; is the frequency modulation speed, f e is the final laser modulation frequency, f s is the initial laser modulation frequency, T is the frequency modulation period; δ 2 (t) is the reflected signal delay; θ 0 is the initial phase; μ is the interference efficiency; H 3 is the correction signal amplitude; H 4 is the measured signal amplitude; δ 3 To correct the signal delay; δ 4 To measure signal delay;

[0064] In particular, in order to filter out high-order secondary signals and DC interference in the interference signal, a DC filter is also provided between the coherent detector and the collector, and between the interferometer and the collector in the present application.

[0065] Step 3: Decomposition and simplification of the computational model

[0066] The instantaneous speed of optical frequency modulation is fitted by a polynomial and simplified to obtain the electric field expression output by the laser:

[0067] In the formula, v i is the i-th coefficient of the modulation speed, i = 1, 2, 3; f(t) is the instantaneous optical frequency of the modulation;

[0068] Similarly, the electric field expression of the reflected signal and the current expression of the measured signal are obtained:

[0069]

[0070] Considering that the distance between the object being measured and the drone is generally within 100 meters during the drone inspection process, which is much smaller than the speed of light, the delay in the corresponding space can be ignored, and the electric field expression of the measurement signal can be simplified to:

[0071]

[0072] Then I' 4 (t) = cos[2πδ 4 f(t)];

[0073] The phase of the measured signal can be expressed as: 4(t)=2πf(t)δ 4 ;

[0074] Step 4: Correction based on the simplified computational model

[0075] Considering that the modulation speed is not constant, the electric field expression of the detection signal and the reflected signal can be transcribed as:

[0076] E 1 '(t)=H 1 cos[2πf s (t-δ 1 )+πv(t-δ 1 ) 2 +θ 0 (t)]

[0077] E' 2 (t) = E 1 '(t-δ 2 )

[0078] Based on the photoelectric square law, the corrected current expression of the measured signal is obtained after filtering the DC signal and the unmeasurable signal in the signal:

[0079] I 4 =H 4 cos[2πf s δ 4 +2πv(t)δ 4 t]=H 4 cos[2πδ 4 [f s +v(t)t]=H 4 cos[2πδ 4 f(t)];

[0080] Similarly, the corrected current expression of the correction signal can be obtained: I 3 =H 3 cos[2πδ 3 f(t)];

[0081] The measured signal is sampled twice using the time points of the correction signal peak and trough as the sampling moments. The frequency interval corresponding to adjacent sampling points is: The phase interval corresponding to adjacent sampling points is Δθ = π;

[0082] Then the secondary sampling time f(t) satisfies 2πδ 3 f(t)=π·N,N=1,2,3...;

[0083] After substituting, we get the corrected electrical quantity expression of the measured signal after secondary sampling:

[0084] It can be found from the above formula that the interval between adjacent points of the measured signal in the frequency domain after processing is a fixed value, and the signal frequency after Fourier transform is also a fixed value, thereby avoiding the influence of the nonlinear frequency modulation process on the measurement accuracy. Based on the frequency information of secondary sampling, the distance expression between the target to be measured and the drone can be obtained:

[0085] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application, rather than to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, ordinary technicians in this field should understand that the technical solution of the present application can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present application.

Claims

1. A high-precision distance measurement method for unmanned aerial vehicle inspection, characterized in that: The steps include: Step 1: Configure the laser ranging and calibration system It comprises a laser (1) arranged on a drone, a first beam splitter (2), a second beam splitter (3), a circulator (4), a coherent detector (5), a Mach-Zehnder interferometer (6), and a collector (8); The laser (1) is used to generate a modulated laser signal. The laser signal is split into a detection signal and an auxiliary signal via a first beam splitter (2); the detection signal is split into a measurement signal and an interference signal via a second beam splitter (3); the measurement signal is irradiated to a target to be measured via a circulator (4); a reflected light signal returns to the circulator (4) to obtain a reflected signal; the reflected signal is coherent with the interference signal and then collected by a coherent detector (5) to obtain a measurement signal; the auxiliary signal is passed through a Mach-Zehnder interferometer (6) to obtain a correction signal; the measurement signal and the correction signal are collected by a collector (8); Step 2: Establish the distance measurement and correction system operation model Based on the basic principles of optical frequency modulation and signal interference, we can obtain The electric field expression of the detection signal: E1(t) = H1cos[2πf s t+πvt 2 +θ0(t)] The electric field expression of the reflected signal: E2(t) = H2cos[2πf s (t-δ2)+πv(t-δ2) 2 +θ0(t)] The measurement signal is the interference signal obtained by the coherence of the detection signal and the reflection signal. The correction signal is the interference signal obtained by the auxiliary signal passing through the Mach-Zehnder interferometer (6). Based on the photoelectric square law, it can be obtained: Current expression of the correction signal: I3(t) = H3cos[2πf(t)δ3] Current expression of the measurement signal: I4(t)=H4cos[2πf(t)δ4] Where f(t) = f s +v1t+v2t 2 +v3t 3 , t is the sampling time; H1 is the detection electric field amplitude; H2 is the reflection electric field amplitude; is the frequency modulation speed, f e is the final laser modulation frequency, f s is the initial laser modulation frequency, T is the frequency modulation period; δ2(t) is the reflected signal delay; θ0 is the initial phase; μ is the interference efficiency; H3 is the correction signal amplitude; H4 is the measurement signal amplitude; δ3 is the correction signal delay; δ4 is the measurement signal delay; Step 3: Decomposition and simplification of the computational model The instantaneous speed of optical frequency modulation is fitted by a polynomial and simplified to obtain the electric field expression output by the laser: Where i is the frequency modulation speed v i The i-th order coefficient, i = 1, 2, 3; f(t) is the instantaneous optical frequency of modulation; Similarly, the electric field expression of the reflected signal and the current expression of the measured signal are obtained: Considering that the distance between the object being measured and the drone during the drone inspection is generally within 100 meters, which is much smaller than the speed of light, the delay in the corresponding space is ignored, and the electric field expression of the measurement signal is simplified to: Then I'4(t)=cos[2πδ4f(t)]; The phase of the measured signal is expressed as: θ4(t) = 2πf(t)δ4; Step 4: Correction based on the simplified computational model Considering that the modulation speed is not constant, the electric field expression of the detection signal and the reflected signal is transcribed as: E’1(t)=H1cos[2πf s (t-δ1)+πv(t-δ1) 2 +θ0(t)] E'2(t)=E'1(t-δ2) Based on the photoelectric square law, the corrected current expression of the measured signal is obtained after filtering the DC signal and the unmeasurable signal in the signal: I″4=H4cos[2πf s δ4+2πv(t)δ4t]=H4cos[2πδ4[f s +v(t)t]=H4cos[2πδ4f(t)]; Similarly, the corrected current expression of the correction signal can be obtained: I″3=H3cos[2πδ3f(t)]; The measured signal is sampled twice using the time points of the correction signal peak and trough as the sampling moments. The frequency interval corresponding to adjacent sampling points is: The phase interval corresponding to adjacent sampling points is Δθ = π; Then the secondary sampling time f(t) satisfies 2πδ3f(t)=π·N, N=1,2,3...; After substituting, we get the corrected electrical quantity expression of the measured signal after secondary sampling: The corrected measured distance expression based on the sub-sampling frequency information is obtained:

2. A high-precision distance measurement method for unmanned aerial vehicle inspection according to claim 1, characterized in that: The laser is a frequency modulated laser source.

3. The high-precision distance measurement method for unmanned aerial vehicle inspection according to claim 1 is characterized in that: In the laser distance measurement and correction system, the power division ratio between the first beam splitter (2) and the second beam splitter (3) is 9:

1.

4. The high-precision distance measurement method for unmanned aerial vehicle inspection according to claim 1 is characterized in that: The laser distance measurement and correction system further comprises a collimator (9) arranged on the rear side of the circulator (4).

Images