Full-field Deformation and Vibration Measurement Method and System Based on FMCW LiDAR

Abstract

A method and system for measuring full-field deformation and vibration based on an FMCW lidar. By cyclically scanning and emitting lidar signals with linear frequency modulation, the local oscillator laser signal and the echo laser signal are coherently processed to obtain a laser coherent signal. The frequencies of the laser coherent signals of the effective measurement points are extracted from each measurement point in the full field, and the distances from each measurement point on the surface of the measured object to the signal emission source are calculated, so as to locate the measured target and obtain the point cloud of the measured object. Through non-linear phase demodulation, the phase information of each point in the point cloud, that is, the measurement point, within each cycle of the traversal scan period is extracted from the frequencies of the laser coherent signals of each measurement point. After the phase information unwrapping process, the deformation and vibration displacement information of each measurement point in the full field is obtained. The present invention can achieve fast cyclic scanning full-field displacement measurement, with convenient operation, realize real-time vibration displacement monitoring of a large number of measurement points in the full field, and accurate deformation displacement measurement.

Description

Technical Field

[0001] The present invention relates to a technology in the field of engineering survey, specifically a method and system for measuring full-field deformation and vibration based on a frequency-modulated continuous wave (FMCW) lidar. Background Art

[0002] In the existing methods for monitoring the vibration response of engineering structures, in the contact measurement method based on accelerometers, complex circuit arrangements are required for full-field multi-point synchronous testing. The sensor networking is complex, and the pre-test preparation period is long. At the same time, the low-frequency response of accelerometers is poor, making it difficult to achieve accurate deformation and vibration displacement measurements. The laser Doppler vibration measurement method measures velocity through the Doppler effect and similarly cannot perform accurate deformation and vibration displacement measurements. It is mainly for single-point measurement, and the operation and calibration are complex. In the vision vibration measurement method, the processing of a large amount of image data streams is computationally intensive and time-consuming, making it difficult to achieve real-time vibration measurement. At the same time, the vision vibration measurement accuracy is low, and it is sensitive to conditions such as light. The existing lidars can only achieve ranging and velocity measurement through mixing signal processing. Limited by the bandwidth, they cannot achieve accurate deformation and vibration displacement measurements and synchronous vibration measurements of multiple points / multiple targets. Summary of the Invention

[0003] In view of the above deficiencies in the prior art, the present invention proposes a method and system for measuring full-field deformation and vibration based on an FMCW lidar. By using the phase information of the interference signal to invert the deformation and vibration displacement information, it can be used for synchronous vibration measurement of multiple measurement points in the full field of large structures, and can achieve fast cyclic scanning full-field displacement measurement. The operation is convenient, and there is no need for complex pre-test measurement point arrangement and networking arrangement. While achieving large-field-of-view high-resolution imaging and positioning, it realizes real-time displacement monitoring of a large number of measurement points in the full field and accurate deformation displacement measurement.

[0004] The present invention is realized through the following technical solutions:

[0005] The present invention relates to a method for measuring full-field deformation and vibration based on an FMCW lidar. By cyclically scanning and emitting a linearly frequency-modulated lidar signal, the local oscillator laser signal and the echo laser signal are coherently processed to obtain a laser coherent signal. The frequencies of the laser coherent signals of the effective measurement points are extracted from each measurement point in the full field, and the distances from each measurement point on the surface of the object under test to the signal emission source are calculated, so as to locate the object under test and obtain the point cloud of the object under test. Through non-linear phase demodulation, the phase information of each point in the point cloud, that is, the measurement point, within each cycle of the traversing scan is extracted from the frequencies of the laser coherent signals of each measurement point. After the phase information unwrapping process, the deformation and vibration displacement information of each measurement point in the full field is obtained.

[0006] The so-called cyclic scanning emission means that the FMCW lidar continuously emits a single beam or multiple linearly frequency-modulated laser signals simultaneously through cyclic scanning, and periodically traverses and scans the entire object to be measured.

[0007] For the traversing scan, record the angles of the laser beam relative to the initial position in the horizontal and vertical directions during the scan.

[0008] For the traversing scan, it is scanned by means of, but not limited to, mechanical rotary scanning, MEMS scanning, Flash scanning, fiber optic scanning, and phased array scanning.

[0009] For the coherent processing, the FMCW LiDAR receives the echo laser signal of the target through the laser receiving unit, and eliminates the DC component in the coherent signal after coherent processing with the local oscillator signal, where: coherent laser coherent signal Where: λ c is the wavelength corresponding to the laser center frequency. I1 and I2 are the optical field intensities of the local oscillator and the reflected light beam respectively. Ignoring the influence of quadratic terms, the amplitude of the optical field intensity of the local oscillator signal The amplitude of the optical field intensity of the reflected signal is the time delay, R is the distance between the laser and the measurement point, x(t) is the displacement caused by the target vibration, c is the speed of light, B is the bandwidth of the FMCW laser signal, T s is the time length of a single cyclic traversing scan, f0 is the initial frequency of the transmitted signal, is the initial phase.

[0010] For the so-called effective measurement points, by collecting the laser coherent signals in each direction during the scan of the LiDAR within a single cyclic traversing scan period T, judge whether the intensity of the coherent signals at each measurement point is lower than the threshold, and obtain the effective measurement point signals higher than the threshold, that is, the effective measurement points. Specifically: generate a laser coherent signal matrix Where: M is the number of laser beams emitted by the laser with a uniform linear distribution, and L is the number of linearly frequency-modulated laser signals emitted within a single cyclic traversing scan period.

[0011] The distance from each measurement point on the surface of the object to be measured to the signal emission source Where: f D (q) is the frequency of the laser coherent signal at the effective measurement point I q , and q is the serial number of the measurement point.

[0012] The positioning described above refers to: based on the distances from each measurement point on the surface of the object to be measured to the laser, and then positioning the target to be measured through the measurement point distances and the angle information of the emitted laser beam in the vertical and horizontal directions.

[0013] The point cloud imaging described above refers to: imaging the object to be measured through the point cloud formed by the position information of each measurement point on the surface of the object to be measured.

[0014] For the phase information of each measurement point within each cycle traversal scanning period, the received laser coherent signal is converted into a complex signal H(q, nT s ) = Hilbert(I q , iT), and according to the frequency f D (q) of the laser coherent signal of the q-th measurement point extracted, the phase information of each measurement point within each cycle traversal scanning period is extracted through a non-linear phase demodulation method where: T is the cycle traversal scanning period, angle() is the operation of taking the phase angle of the complex signal, i is the serial number of the cycle traversal scanning period, and q is the target serial number.

[0015] The phase information unwrapping process described above refers to: calculating the phase difference between two adjacent cycle traversal scanning periods before and after When it is greater than π, then add 2π to the phase of the latter cycle traversal scanning period; when it is less than -π, then subtract 2π from the phase of the latter cycle traversal scanning period, and the deformation and vibration displacement information of each measurement point in the whole field is inversely obtained by using the unwrapped phase: where: mean() is to take the mean value, is the included angle between the laser beam line of sight and the deformation and vibration displacement direction.

[0016] Technical effects

[0017] The present invention as a whole solves the deficiencies existing in the prior art in the non-contact measurement of synchronous deformation and vibration at long distances for multiple points / targets, the complex wiring arrangement and sensor networking of contact measurement methods represented by accelerometers, the long preparation period, poor low-frequency response, and the problem that only velocity measurement can be performed in the laser Doppler vibrometry method and accurate deformation and displacement measurement cannot be achieved; the present invention extracts the phase information of each measurement point within each cycle traversal scanning period from the frequency of the laser coherent signal of each measurement point through non-linear phase demodulation, and obtains the deformation and vibration displacement information of each measurement point in the whole field through the phase information unwrapping process. Compared with the prior art, the present invention can achieve accurate deformation and vibration displacement measurement, and secondly, through the scanning method, synchronous measurement of multiple measurement points / targets in the whole field can be realized. Description of the drawings

[0018] Figure 1 is the flow chart of the present invention;

[0019] Figure 2 This is a schematic diagram of the system of the present invention. Detailed implementation manners

[0020] As Figure 2 shown, this embodiment relates to a full-field deformation and vibration measurement system based on an FMCW lidar, including: an FMCW lidar module, a signal acquisition module, a target detection, positioning and imaging module, a deformation and vibration displacement extraction module, and a display and storage module, wherein: The FMCW lidar module performs full-field scanning on the object to be measured, acquires the echo laser signal and outputs a laser coherent signal. The signal acquisition module transmits the acquired laser coherent signal to the target detection, positioning and imaging module and the deformation and vibration displacement extraction module. The target detection, positioning and imaging module processes the laser coherent signal, performs target detection, then locates the target according to the laser direction and detection distance information, and finally images the measured target according to the target point cloud information; The deformation and vibration displacement extraction module extracts the deformation and vibration displacement information of the point to be measured according to the target position information obtained by the target detection, positioning and imaging module and the laser coherent signal acquired during the scanning process; The display and storage module is used to display the target point cloud imaging result and the deformation and vibration displacement information, and save them.

[0021] The described FMCW lidar module includes: a laser transmitter, a laser receiver, an interference receiving unit, and a scanning control unit, wherein: The laser transmitter divides the linearly frequency-modulated laser source signal into two paths, one path is used as the local oscillator signal, and the other path is used as the transmitted signal for target detection and perception. The laser receiver receives the laser signal reflected by the target; The interference receiving unit performs coherent processing on the local oscillator signal and the received target-emitted laser signal, and obtains the laser coherent signal; The scanning control unit controls the direction of the laser beam to realize full-field scanning of the target.

[0022] The described laser transmitter includes one or more laser emission probes.

[0023] The full-field deformation and vibration measurement system further includes a display and storage module, which is connected to the target detection, positioning and imaging module and the deformation and vibration displacement extraction module, and is used to display the target positioning information, the point cloud imaging result, and the phase change and vibration displacement test result, and save the above results.

[0024] As Figure 1 shown, this embodiment is a full-field deformation and vibration measurement method based on the above system, including the following steps:

[0025] Step 1, circularly scan and transmit and receive the linearly frequency-modulated lidar signal;

[0026] Step 1.1, The FMCW lidar continuously emits one or multiple linearly frequency-modulated laser signals simultaneously;

[0027] Step 1.2, By means of cyclic scanning, the laser signal traverses the entire object to be measured with a period T, and the angles of the laser beam relative to the initial position in the horizontal and vertical directions are recorded during the scanning process;

[0028] Step 1.3, The FMCW LiDAR receives the reflected signal of the target through the laser receiving unit.

[0029] Step 2, The local oscillator laser signal and the received echo laser signal are subjected to coherent processing to obtain a laser coherent signal;

[0030] Step 2.1, The received target reflection signal and the local oscillator signal are subjected to coherent processing in the interference receiving unit.

[0031] Step 2.2, A mixed-frequency laser signal is obtained through coherent processing, and the coherent form of its optical field intensity is:

[0032] where: I1 and I2 are the optical field intensities of the local oscillator and the reflected light beam respectively. Ignoring the influence of quadratic terms, the coherent laser coherent signal is: where: λ c is the wavelength corresponding to the laser center frequency.

[0033] Step 2.3, The DC components I1 and I2 in the coherent signal are eliminated to obtain a laser coherent signal component that only contains the target motion information

[0034] Step 3, Detection, positioning and imaging of each measurement point or target in the whole field, including the following steps:

[0035] Step 3.1, Collect the laser coherent signals in all directions during a cyclic traversal scanning period T of the LiDAR during the scanning process: where: M is the number of laser beams emitted by the laser with a uniform linear distribution, and L is the number of linearly frequency-modulated laser signals emitted within a single cyclic traversal scanning period.

[0036] Step 3.2, Determine whether the intensity of the coherent signal at each measurement point is lower than the threshold, obtain the effective measurement point signal higher than the threshold, and extract the frequency f q of the laser coherent signal at the effective measurement point I D (q), where q is the serial number of the measurement point.

[0037] Step 3.3: Calculate the distances from each measurement point on the object surface to the laser, and then locate the measured target based on the measurement point distances and the angular information of the emitted laser beam in the vertical and horizontal directions;

[0038] Step 3.4: Image the measured object through the point cloud formed by the position information of each measurement point on the surface of the measured object.

[0039] The method for extracting the deformation and vibration displacement information of each measurement point or the measured target described in Step 4 is characterized by including the following steps:

[0040] Step 4.1: Convert the received laser coherent signal into a complex signal through Hilbert transform;

[0041] Step 4.2: According to the frequencies of the laser coherent signals of each measurement point extracted in Step 3.2, extract the phase information of each measurement point within each cycle-by-cycle scanning period through a non-linear phase demodulation method;

[0042] Step 4.3: Since the laser wavelength is very short, a small displacement will cause a large phase change. Therefore, phase jumps may occur in the demodulated phases within adjacent cycle-by-cycle scanning periods. It is necessary to perform a phase unwrapping operation on the extracted phase information to reduce the mutation measurement error. Specifically: Calculate the phase difference between the previous and the next adjacent cycle-by-cycle scanning periods When it is greater than π, set the phase of the next cycle-by-cycle scanning period to be added with 2π; when it is less than -π, set the phase of the next cycle-by-cycle scanning period to be subtracted by 2π.

[0043] Step 4.4: Extraction of the deformation and vibration displacement information of each measurement point in the whole field: Use the unwrapped phase to inversely calculate the deformation and vibration displacement information of each measurement point in the whole field:

[0044] Compared with the prior art, this method can achieve accurate full-field multi-measurement point deformation and vibration displacement measurement.

[0045] The above specific implementation can be locally adjusted by those skilled in the art in different ways without departing from the principles and purposes of the present invention. The protection scope of the present invention is subject to the claims and is not limited by the above specific implementation. All implementation solutions within its scope are subject to the present invention.

Claims

1. A method for measuring full-field deformation and vibration based on FMCW lidar, characterized in that, By circularly scanning to emit a lidar signal with linear frequency modulation, performing coherent processing on the local oscillator laser signal and the echo laser signal to obtain a laser coherent signal; extracting the frequencies of the laser coherent signals of the effective measurement points from all measurement points in the entire field, calculating the distances from each measurement point on the surface of the measured object to the signal emission source, thereby positioning the measured target and obtaining the point cloud of the measured object; extracting the phase information of each point in the point cloud within each cycle of the traversal scan period from the laser coherent signals of each measurement point through non-linear phase demodulation, and obtaining the deformation and vibration displacement information of all measurement points in the entire field through phase information unwrapping processing.

2. The method for measuring full-field deformation and vibration based on FMCW lidar according to claim 1, characterized in that The circular scanning emission mentioned above means that: the FMCW lidar continuously emits a beam or multiple beams of laser signals with linear frequency modulation in a circular scanning manner, periodically traversing the entire measured object and recording the angles of the laser beam relative to the initial position in the horizontal and vertical directions during the scanning process.

3. The method for measuring full-field deformation and vibration based on an FMCW lidar according to claim 1, wherein For the coherent processing, the FMCW LiDAR is used to receive the echo laser signal of the target through the laser receiving unit, and after coherent processing with the local oscillator signal, the DC component in the coherent signal is eliminated, where: coherent laser coherent signal Where: λ c is the wavelength corresponding to the laser center frequency, I1 and I2 are the optical field intensities of the local oscillator and the reflected light beam respectively. Ignoring the influence of the quadratic term, the amplitude of the optical field intensity of the local oscillator signal The amplitude of the optical field intensity of the reflected signal is the time delay, R is the distance between the laser and the measurement point, x(t) is the displacement caused by the vibration of the target, c is the speed of light, B is the bandwidth of the FMCW laser signal, T s is the length of a single scan time, f0 is the initial frequency of the transmitted signal, is the initial phase.

4. The method for measuring full-field deformation and vibration based on an FMCW lidar according to claim 1, characterized in that, The effective measurement points are obtained by collecting the laser coherence signals in each direction during a cyclic traversal scanning period T in the LiDAR scanning process, and determining whether the intensity of the coherence signals at each measurement point is lower than the threshold value, so as to obtain the effective measurement point signals higher than the threshold value, that is, the effective measurement points. Specifically: generate a laser coherence signal matrix where: M is the number of laser beams emitted by the laser with a uniform linear distribution, and L is the number of linearly frequency-modulated laser signals emitted during a single cyclic traversal scanning period.

5. The method for measuring full-field deformation and vibration based on an FMCW lidar according to claim 1, wherein The distances from each measuring point on the surface of the object under test to the signal emitter where: f D (q) is the frequency of the laser coherent signal at the effective measuring point I q and q is the serial number of the measuring point; The positioning mentioned above means that: based on the distances from each measurement point on the surface of the object to be measured to the laser, and then positioning the measured target through the measurement point distances and the angle information of the emitted laser beam in the vertical and horizontal directions.

6. The method for measuring full-field deformation and vibration based on an FMCW lidar according to claim 1, characterized in that, Each of the above-mentioned cycles traverses the phase information of each measurement point within the scanning period, and the received laser coherent signal is converted into a complex signal H(q, nT s ) = Hilbert(I q , iT), and according to the frequency f D (q) of the laser coherent signal of the q-th measurement point extracted, the phase information of each measurement point within each cycle traversing the scanning period is extracted through the non-linear phase demodulation method where: T is the cycle traversing scanning period, angle() is the operation of taking the phase angle of the complex signal, i is the serial number of the cycle traversing scanning period, and q is the target serial number.

7. The method for measuring full-field deformation and vibration based on FMCW lidar according to claim 1, characterized in that, The phase information unwrapping process mentioned above refers to: calculating the phase difference between two adjacent cycle traversal scanning periods before and after When it is greater than π, the phase of the latter cycle traversal scanning period is added with 2π; when it is less than -π, the phase of the latter cycle traversal scanning period is subtracted by 2π, and the deformation and vibration displacement information of each measurement point in the whole field is obtained by inverting the unwrapped phase: where: mean() is to take the mean value, is the three-dimensional space angle between the laser beam line of sight and the deformation and vibration displacement direction.

8. A full-field deformation and vibration measurement system based on an FMCW lidar for implementing the method according to any one of claims 1 to 7, characterized in that, It includes: An FMCW lidar module, a signal acquisition module, a target detection, positioning and imaging module, a deformation and vibration displacement extraction module, and a display and storage module. Among them: the FMCW lidar module performs full-field scanning on the object to be measured, collects the echo laser signal and outputs the laser coherent signal. The signal acquisition module transmits the collected laser coherent signal to the target detection, positioning and imaging module and the deformation and vibration displacement extraction module. The target detection, positioning and imaging module processes the laser coherent signal, performs target detection, then positions the target according to the laser direction and detection distance information, and finally images the measured target according to the target point cloud information. The deformation and vibration displacement extraction module extracts the deformation and vibration displacement information of the measurement points to be measured according to the target position information obtained by the target detection, positioning and imaging module and the laser coherent signal collected during the scanning process. The display and storage module is used to display the target point cloud imaging result and the deformation and vibration displacement information, and perform storage.

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