A vibration velocity demodulation system and method for a laser Doppler vibrometer

Abstract

The present application relates to a kind of vibration velocity demodulation systems of laser doppler vibrometer, the laser doppler vibrometer includes laser vibration measurement and laser ranging two modes, the probe of laser vibration measurement mode is converted into beat frequency electric signal after the frequency shift laser interference signal collected and exports, including high-precision heterodyne conditioning circuit, the down-conversion module with multiplier, high-speed ADC circuit and high-speed acquisition and demodulation module, its down-conversion module includes analog multiplier, first switch, second switch and the measurement channel of different down-conversion frequency suitable for low vibration speed, the first input end of analog multiplier is connected after the intermediate frequency signal processed by high-precision heterodyne conditioning circuit, other end is connected with second switch.This application simultaneously provides the vibration velocity demodulation method for realizing the vibration velocity demodulation system of the described.

Description

Technical Field

[0001] This invention relates to a vibration velocity demodulation system and demodulation method for a laser Doppler vibration meter. Background Technology

[0002] Vibration information from mechanical equipment is frequently used to assess its operational status and diagnose malfunctions. Therefore, accurate vibration measurement is crucial for ensuring safe mechanical operation. Vibration measurement can be categorized into contact and non-contact methods. Contact measurement requires attaching a vibration sensor to the surface of the object being measured, but the added mass often disrupts the original vibration state of the object, affecting measurement accuracy. Laser Doppler vibration meters can perform high-precision non-contact optical measurements of surface vibration velocity and amplitude, and are widely used in aerospace, precision manufacturing, structural health monitoring, and life sciences.

[0003] Laser Doppler vibrometers are mainly classified into single-point laser vibrometers, multi-point laser vibrometers, scanning laser vibrometers, three-dimensional laser vibrometers, and three-dimensional scanning laser vibrometers. Three-dimensional laser Doppler vibrometers can be used in conjunction with finite element models to perform finite element vibration measurement and analysis on geometrically irregular objects, providing a more comprehensive understanding of the object's inherent characteristics. Three-dimensional laser Doppler vibrometers can effectively avoid the influence of in-plane vibrations and offer high measurement accuracy.

[0004] The prior art patent application number 201611164666.6 discloses a heterodyne laser vibrometer method based on bandpass sampling. This method proposes an automatic selection method for the optimal sampling frequency based on bandpass sampling to determine the optimal sampling frequency for the heterodyne laser Doppler signal, and demodulates the acquired heterodyne laser Doppler signal based on the Phase Unfolded Sine Approximation Method (PUSAM). This algorithm effectively solves the shortcomings of traditional heterodyne laser vibrometer methods, which require high sampling frequencies or have phase delays for heterodyne laser vibrometer measurements. However, compared to directly calling the filter module, it is more complex, and the bandpass sampling accuracy of this method is not high, which cannot meet the requirements of high-precision vibrometer measurement. Summary of the Invention

[0005] The purpose of this invention is to provide a vibration velocity demodulation system and method for a laser Doppler vibration meter, enabling the laser three-dimensional vibration measurement system to quickly and accurately demodulate the vibration velocity of an object. The technical solution is as follows:

[0006] A vibration velocity demodulation system for a laser Doppler vibrometer, wherein the laser Doppler vibrometer includes two modes: laser vibration measurement and laser ranging. In the laser vibration measurement mode, the probe converts the acquired frequency-shifted laser interference signal into a beat frequency electrical signal and outputs it. The system is characterized by including a high-precision heterodyne conditioning circuit, a down-conversion module with a multiplier, a high-speed ADC circuit, and a high-speed acquisition and demodulation module.

[0007] The high-precision heterodyne conditioning circuit takes a beat frequency electrical signal converted from a frequency-shifted laser interference signal as its input, and passes through a high-pass filter and amplification circuit, a limiting circuit, and a low-pass filter circuit in sequence. The output is a processed intermediate frequency signal.

[0008] The downconversion module includes an analog multiplier, a first switch, a second switch, and measurement channels for different downconversion frequencies suitable for medium and low vibration velocities. The first input terminal of the analog multiplier is connected to the intermediate frequency signal processed by a high-precision heterodyne conditioning circuit, and the other end is connected to the second switch.

[0009] The FPGA is used to generate multiple square waves of different frequencies and one unit step signal to form a measurement channel with different down-conversion frequencies suitable for medium and low vibration velocities. The multiple square waves are processed by their respective bandpass filters to obtain sine waves of different frequencies. The unit step signal generated by the FPGA or the sine waves of different frequencies processed by the bandpass filter can be selectively connected to the second input terminal of the multiplier through the first switch.

[0010] The output of the multiplier can be selectively connected to the first measurement channel via a second switch, or connected to a measurement channel with a measurement frequency lower than that of the first measurement channel after passing through a corresponding four-way low-pass filter. The output of each measurement channel is sequentially passed through a high-speed ADC circuit and a high-speed acquisition and demodulation module to obtain the vibration velocity.

[0011] Furthermore, the first measurement channel is a 50MHz measurement channel.

[0012] Furthermore, the square wave is four-channel, generated by the internal crystal oscillator of the FPGA with frequencies of 49MHz, 45MHz, 40MHz and 25MHz respectively; the measurement channels of different frequencies include a 25MHz measurement channel, a 10MHz measurement channel, a 5MHz measurement channel and a 1MHz measurement channel.

[0013] This invention also provides a vibration velocity demodulation method using the aforementioned vibration velocity demodulation system, comprising the following steps:

[0014] (1) By selecting the two switches, the unit step signal and the first measurement channel are connected without down-conversion to obtain the initial vibration velocity value;

[0015] (2) Based on the initial vibration velocity value, vibration velocity measurements are performed under the following different conditions:

[0016] 1) When the initial vibration velocity value is between 7.5m / s and 12m / s, make the vibration meter work in the first measurement channel and set the sampling ratio to 1;

[0017] 2) When the initial vibration velocity value is between 3m / s and 7.5m / s, the vibration meter operates in the 25MHz measurement channel and the sampling factor is set to 1.

[0018] 3) When the initial vibration velocity value is between 1m / s and 3m / s, the vibration meter operates in the 10MHz measurement channel and the sampling rate is set to 2.

[0019] 4) When the initial vibration velocity value is between 0.1m / s and 1m / s, the vibration meter operates in the 5MHz measurement channel and the sampling rate is set to 4.

[0020] 5) When the initial vibration velocity value is less than 0.1 m / s, the vibration meter is working in the 1MHz measurement channel and the sampling rate is set to 6.

[0021] 6) Demodulate and obtain the vibration velocity;

[0022] (3) If the vibration meter is working in the 1MHz measurement channel and the measured vibration velocity is less than 0.02m / s, use the optical switch to switch the probe to the laser ranging mode, use the laser ranging module to measure the vibration displacement of the measured object within a fixed time interval, and directly obtain the vibration velocity corresponding to the extremely low vibration frequency.

[0023] Furthermore, the method for demodulating and obtaining the vibration velocity is as follows:

[0024] 1) The high-speed ADC circuit acquires the signal after down-conversion by the analog multiplier at a sampling frequency of 250Msps;

[0025] 2) The numerically controlled oscillator (NCO) generates two orthogonal signals, the frequency of which is the difference between the center frequency of the acousto-optic frequency shifter and the down-conversion frequency of the analog multiplier.

[0026] 3) The signal acquired by the high-speed ADC circuit is multiplied with the quadrature signal generated by the NCO, and after low-pass filtering, the I and Q signals are obtained. At this time, the frequencies of the two signals are down-converted by the multiplier and down-converted by the demodulation, and only contain frequency offset information, which is in an oversampling state.

[0027] 4) The CIC decimation filter and HB decimation filter are cascaded to match the previous analog multiplier down-conversion measurement channel for different vibration velocities.

[0028] 5) Divide the extracted I and Q signals, and then use the CORDIC algorithm to perform arctangent calculation to obtain the instantaneous displacement of the measured object;

[0029] 6) Obtain the vibration velocity by differentiating the displacement, and simultaneously plot the frequency discrimination characteristic curve;

[0030] 7) Divide the filtered frequency discrimination curve into 16 intervals. Each interval corresponds to the smaller demodulated vibration velocity and the actual vibration velocity, and there is a corresponding interpolation formula. Calculate the filtered frequency discrimination voltage value into the final demodulation speed, select the corresponding interpolation formula, substitute the demodulated vibration velocity into the interpolation formula to calculate the actual vibration velocity, and complete the measurement of vibration velocity.

[0031] The beneficial effects of this invention are as follows:

[0032] 1. Laser Doppler technology cannot measure vibration velocities with frequencies close to zero. When the vibration frequency is extremely low, the velocity is very low, and the corresponding Doppler frequency shift is very small. During subsequent demodulation, the I and Q signals have very low frequencies, long periods, and small, slow frequency changes, easily exceeding the device's frequency resolution range. To accurately measure these small frequency changes, longer data acquisition times are needed to accumulate enough data points to improve frequency resolution. This increases demodulation time, compromising real-time performance. Furthermore, calculating multiple points consumes significant FPGA resources. Using a laser ranging module to measure the vibration displacement of the object within a fixed time interval directly yields the velocity corresponding to extremely low vibration frequencies.

[0033] 2. This invention utilizes the down-conversion principle of a multiplier to match channels with different decimation rates. While satisfying the Nyquist sampling theorem, it down-converts and decimates high-sampling-rate signals, reducing the signal sampling rate and mitigating the impact of frequency offset errors caused by high-speed ADC quantization errors. This facilitates the manufacture of a low-cost, high-precision vibration velocity demodulation module. Furthermore, this invention reduces the computational load of subsequent FPGA arctangent calculations while ensuring that the decimated signal can be completely recovered to the original signal without information loss, thus guaranteeing the real-time performance of vibration velocity measurement.

[0034] 3. This invention proposes a novel digital demodulation system for velocity demodulation in laser Doppler vibrometers. After orthogonal digital down-conversion, a zero-IF frequency offset signal is obtained. At this point, the signal sampling rate remains the same as that of the high-speed ADC module, resulting in a high data rate. However, the signal bandwidth is much smaller than the sampling rate. Therefore, filtering and decimation are used to reduce the sampling frequency and prevent aliasing in the spectrum, allowing for the acquisition of a complete and effective frequency offset signal at a lower sampling rate. To facilitate subsequent processing, the CORDIC algorithm is used to solve the arctangent, ensuring accuracy while meeting the system's high-speed demodulation requirements. Frequency discrimination curve interpolation is used to correct demodulated waveform distortion that occurs when the velocity and frequency offset are large, further improving the accuracy of velocity demodulation. Attached Figure Description

[0035] Figure 1 Overall measurement scheme for laser three-dimensional vibration meter

[0036] Figure 2 Structure of a laser Doppler vibration meter probe

[0037] Figure 3 Optical switch switching module

[0038] Figure 4 A schematic diagram of a down-conversion analog multiplier. Detailed Implementation

[0039] The present invention will be described below in conjunction with several embodiments.

[0040] 1. Overall Structure

[0041] The overall measurement scheme of the laser three-dimensional vibration meter is as follows: Figure 1 As shown, three independent laser vibration probes are used, namely the main probe, sub-probe 1 and sub-probe 2. The sub-probe includes two parts: an optical path and an electronic control system. The main probe includes four parts: an optical path, an electronic control system, a camera and a video transmission interface. The camera is used to correspond between the image coordinate system and the world coordinate system during the 3D scanning process.

[0042] Laser Doppler vibration meter probe structure as follows Figure 2 As shown, a helium-neon laser emits laser light, which is split into two parts (1:9) by fiber coupler OC1. One part, containing 90% of the energy, is sent to the sample arm, and the other part, containing 10%, is sent to the reference arm. An acousto-optic frequency shifter is used to externally modulate the optical signal in the reference arm, shifting its frequency by the same amount as the frequency of the external radio frequency drive signal applied to the acousto-optic frequency shifter. The frequency of the radio frequency drive signal is set to f. AOM Therefore, the frequency f of the optical signal in the reference arm s Change to f s ':

[0043] f s '=f s +f AOM

[0044] The frequency of the optical signal in the other sample arm is f sThe optical signal from the sample arm is fed into port 1 of the circulator and output from port 2. The other end of port 2 is connected to an optical switch, allowing switching between the laser vibration measurement module and the laser ranging module; both modules share the subsequent optical path. If the optical switch is connected to the laser ranging module, the light emitted by the laser ranging module is coupled into the optical fiber via a fiber collimator, then becomes parallel light again via the optical switch and another fiber collimator. After being reflected by a lens fixed on the two-dimensional angular displacement stage, it strikes the sample surface and returns along the same path. The main control computer controls the rotation angle of the two-dimensional angular displacement stage through a microcontroller in the front-end real-time processing system. The angular displacement information is transmitted to the angular displacement stage controller via the interface and power supply circuit module, thereby controlling the rotation of the angular displacement stage. This causes the laser spot on the object surface to move along a certain trajectory, allowing measurement of vibration velocity and displacement at multiple locations, achieving scanning vibration measurement of the workpiece surface. The laser ranging module receives the reflected light and calculates the distance between the probe and the sample, inputting this distance to the main control computer via the interface and power supply circuit module to assist in alignment, scanning, and other operations. If the optical switch is connected to port 2 of the circulator, the laser output from port 2 is transformed into a parallel beam by the fiber collimator. The beam emitted by the laser vibration module is reflected by a lens fixed on the two-dimensional angular displacement stage and illuminates the sample surface. The sample moves at a certain speed, and the frequency of the light reflected from the sample undergoes a Doppler frequency shift f. The reflected light returns along the original path, is emitted by the lens on the two-dimensional angular displacement stage, and is then coupled into the fiber optic cable by the fiber collimator. It is input from port 2 of the circulator and output from port 3. Therefore, the frequency f of the optical signal output from port 3 is... s "for:

[0045] f s =f s +f

[0046] The optical signals from the sample arm and the reference arm are combined via fiber coupler OC2. The combined optical signal and the light from the reference arm are then input to a balanced photodetector, where they are converted into electrical signals with a frequency of:

[0047] f = f s '-f s =f AOM -f

[0048] This signal is then sent to a high-precision heterodyne conditioning circuit and a demodulation circuit for vibration velocity demodulation.

[0049] 2. Measure vibration velocity at extremely low vibration frequencies using a laser ranging module.

[0050] The laser Doppler principle cannot measure vibration velocities with frequencies close to zero. When the vibration frequency is extremely low, the velocity is very low, and the corresponding Doppler frequency shift is very small. During subsequent demodulation, the I and Q signals have very low frequencies, long periods, and small, slow frequency changes, easily exceeding the device's frequency resolution range. To accurately measure these small frequency changes, longer data acquisition times are needed to accumulate enough data points to improve frequency resolution. This increases demodulation time, compromising real-time performance. Furthermore, calculating multiple points consumes significant FPGA resources. Therefore, an optical switch is used to switch the probe to laser ranging mode, such as... Figure 3 As shown, a laser ranging module is used to measure the vibration displacement of the object under test within a fixed time interval, directly obtaining the vibration velocity corresponding to extremely low vibration frequencies, thus rapidly measuring the vibration velocity while ensuring accuracy. An optical switch allows the laser vibration probe to switch between laser Doppler vibration velocity measurement mode and laser ranging vibration velocity measurement mode. The laser ranging module and the laser ranging module share the subsequent measurement optical path. If the optical switch is connected to the laser ranging module, the light emitted by the laser ranging module is coupled into the optical fiber through an optical fiber collimator, then expanded into parallel light by the optical switch and another optical fiber collimator. After being reflected by a lens fixed on a two-dimensional angular displacement stage, it strikes the sample surface and returns along the same path. The laser ranging module receives the reflected light and calculates the distance between the probe and the sample, denoted as S1. After a fixed time interval Δt, the laser ranging module measures the distance between the probe and the sample again, denoted as S2. The vibration velocity of the object under test at this time is then calculated. It can be obtained directly without demodulation.

[0051] 3. Analog multiplier down-conversion

[0052] A balanced photodetector converts the frequency-shifted laser interference signal into a beat frequency electrical signal, which is then processed by a high-precision heterodyne conditioning circuit. This circuit filters out the DC component and low-frequency interference signals from the photodetector output signal. A limiting circuit then compresses the amplitude modulation range to suppress amplitude modulation noise. Finally, a low-pass filter, high-pass filter, amplification circuit, and limiting circuit further filter out high-frequency noise and prevent aliasing. The processed frequency signal is relatively high, which is not conducive to subsequent demodulation. Therefore, an analog multiplier down-conversion module is provided. An FPGA generates square waves and step signals to form measurement channels with different down-conversion frequencies suitable for low to medium vibration velocities (less than 7.5 m / s). These channels are matched with subsequent channels of different decimation rates. Under the premise of satisfying the Nyquist sampling theorem, the high sampling rate signal is down-converted and decimated, reducing the sampling rate and mitigating the impact of frequency offset errors caused by subsequent high-speed ADC quantization errors. This facilitates the manufacture of a low-cost, high-precision vibration velocity demodulation module. Figure 4As shown, an FPGA is used to generate four square waves with frequencies of 49MHz, 45MHz, 40MHz, and 25MHz, and one unit step signal, respectively, to form measurement channels with different ranges. The square waves are converted into sine waves after passing through a bandpass filter and are connected to a switch at one end of the multiplier. One end of the multiplier receives the frequency signal processed by a high-precision heterodyne conditioning circuit, and the other end is connected to the switch. The switch can select to receive a unit step signal, a 49MHz sine wave, a 45MHz sine wave, a 40MHz sine wave, or a 25MHz sine wave. It is important to note that for the subsequent digital demodulation process to proceed smoothly, the down-conversion frequency of the multiplier must be greater than the Doppler frequency shift generated by the vibration of the measured object; otherwise, spectral aliasing will occur during subsequent quadrature down-conversion digital demodulation. Therefore, the maximum Doppler frequency shift of the measured object applicable to the analog multiplier down-conversion is 25MHz, corresponding to a vibration velocity of approximately 7.9m / s. Thus, this method is only suitable for low to medium vibration velocities less than 7.5m / s. When the vibration velocity of the workpiece under test is relatively high, between 7.5 m / s and 12 m / s, the Doppler frequency shift is between 23.7 MHz and 37.9 MHz. A unit step signal is connected to the switch, and to meet the measurement range, no down-conversion is performed; the vibration meter operates in the 50 MHz measurement channel. When the vibration velocity of the workpiece under test is between 3 m / s and 7.5 m / s, the corresponding Doppler frequency shift is between 9.5 MHz and 23.7 MHz. A 25 MHz sine wave is connected to the switch, and the multiplier down-converts the frequency signal processed by the high-precision heterodyne conditioning circuit, shifting the frequency down by 25 MHz. The vibration meter operates in the 25 MHz measurement channel. When the vibration velocity of the workpiece under test is between 1 m / s and 3 m / s, the corresponding Doppler frequency shift is between 3.16 MHz and 9.5 MHz. A 40 MHz sine wave is connected to the switch, and the multiplier down-converts the frequency signal processed by the high-precision heterodyne conditioning circuit. The frequency signal processed by the heterodyne conditioning circuit is down-converted, shifting the frequency down by 40MHz. At this time, the vibration meter operates in the 10MHz measurement channel. When the vibration velocity of the workpiece under test is between 0.1m / s and 1m / s, the corresponding Doppler frequency shift is between 0.32MHz and 3.16MHz. At this time, a 45MHz sine wave is connected to the switch, and the multiplier down-converts the frequency signal processed by the high-precision heterodyne conditioning circuit, shifting the frequency down by 45MHz. At this time, the vibration meter operates in the 5MHz measurement channel. When the vibration velocity of the workpiece under test is less than 0.1m / s, the corresponding Doppler frequency shift is less than 0.32MHz. At this time, a 49MHz sine wave is connected to the switch, and the multiplier down-converts the frequency signal processed by the high-precision heterodyne conditioning circuit, shifting the frequency down by 49MHz. At this time, the vibration meter operates in the 1MHz measurement channel to measure low vibration velocities.The down-conversion measurement channel shifts the frequency signal processed by the high-precision heterodyne conditioning circuit, resulting in a double-sideband modulated signal. This signal needs to be processed by a low-pass filter to obtain the down-shifted signal, which is then input to the high-speed ADC circuit for data acquisition. The intermediate frequency signal after down-conversion by the analog multiplier is processed by high-speed ADC sampling, digital demodulation, and interpolation to obtain the vibration velocity corresponding to the Doppler frequency shift. The analog multiplier down-conversion reduces the computational load of subsequent FPGA arctangent calculations while ensuring that the extracted signal can be completely recovered to the original signal without information loss, thus guaranteeing the real-time performance of vibration velocity measurement.

[0053] 4. Implementation of the digital demodulation system

[0054] The digital demodulation system is implemented on an FPGA platform. The first step is to digitally sample the beat frequency signal after down-conversion by the analog multiplier. The analog signal is then converted into a digital signal by a high-speed ADC circuit. Next, an NCO (numerically controlled oscillator) generates two signals with a frequency difference of 90° between the center frequency of the acousto-optic frequency shifter and the down-conversion frequency of the analog multiplier. These signals are multiplied by the demodulated signal, and the product is then passed through a low-pass filter to filter out out-of-band signals, down-converting the signal to the baseband signal. However, since the sampling rate of the high-speed ADC is usually quite high, the baseband signal after down-conversion is in a state of severe oversampling, which does not meet the requirements of subsequent baseband processing. It is necessary to reduce the data rate of the I and Q signals without distortion. Based on the different measurement channels formed by the down-conversion of the analog multiplier, different decimation ratios are designed to correspond to them. After CIC filtering and HB filtering, the baseband signal is reduced to a lower data rate, which is suitable for the subsequent signal processing. Divide the extracted I and Q values, and use the CORDIC arctangent algorithm to obtain the instantaneous vibration displacement of the measured object. The vibration velocity of the measured object can be obtained by differentiating the displacement.

[0055] Assuming the target vibrates sinusoidally, its displacement s(t) is expressed as follows:

[0056] s(t)=Asin(2πft)

[0057] In the formula, A represents the amplitude of the object's vibration, and f represents the frequency of the object's vibration. Therefore, the object's vibration velocity v(t) is:

[0058] v(t) = 2πfAcos(2πft)

[0059] Therefore, the Doppler frequency shift caused by the vibration of the object is:

[0060]

[0061] The Doppler frequency shift obtained after frequency shifting via the acousto-optic frequency shifter and optical coupler is:

[0062]

[0063] The carrier voltage signal obtained by the balanced photodetector is:

[0064]

[0065] The down-converted signal is multiplied by the carrier signal using an analog multiplier to obtain a mixed signal. This mixed signal is then limited by a frequency-limited low-pass filter to obtain the shifted carrier signal. The down-conversion frequency is set to f. c .

[0066]

[0067] After filtering out high-frequency signals, the carrier signal obtained after down-conversion by the analog multiplier is obtained, i.e.

[0068]

[0069] During demodulation, an NCO (numerically controlled oscillator) generates two mixed signals with a center frequency and a down-conversion frequency from the multiplier. The frequency difference between the center frequency of the acousto-optic frequency shifter and the down-conversion frequency of the analog multiplier, with a phase difference of 90°, is used. These signals are multiplied by St'(t), and the product is then passed through a low-pass filter to remove out-of-band signals. The signal is down-converted to the baseband signal, leaving the integral sum of the instantaneous frequency offset (low-frequency signal). For coherent demodulation, generating a signal orthogonal to the carrier signal is crucial. Using an NCO ensures that the generated signal is strictly orthogonal to the carrier signal and has high accuracy.

[0070] The sinusoidal signal mixed by the center frequency and the down-conversion frequency of the multiplier is as follows:

[0071] f sin (t)=sin(2π(f AOM -f c )t)

[0072] The cosine signal of the mixture between the center frequency and the down-conversion frequency of the multiplier is as follows:

[0073] f cos (t)=cos(2π(f AOM -f c )t)

[0074] After quadrature demodulation, the I and Q split signals are obtained:

[0075]

[0076]

[0077] After filtering by the low-pass filter, only the low-frequency terms remain, i.e., I and Q are:

[0078]

[0079]

[0080] Because high-speed ADCs typically have high sampling rates, the I and Q signals after down-conversion are in an oversampled state. Therefore, it's necessary to reduce the data rates of both the I and Q signals without distortion. A cascaded CIC decimation filter and an HB decimation filter are used. When implementing decimation, the decimation factor of the CIC filter and the number of cascaded stages of the HB filter are both related to the resolution setting. The HB filter is the second stage of the decimation filter; it is essentially an FIR filter and can be implemented using the FIR Compiler IP core. The FIR Compiler is an FIR filter design module provided by ISE, allowing users to set the filter type and import filter coefficients.

[0081] Based on the different measurement channels formed by the downconversion of the analog multiplier, different decimation ratios were designed to correspond to these channels. Simulation experiments showed that when the vibration velocity of the workpiece is relatively high, between 7.5 m / s and 12 m / s, the Doppler frequency shift is between 23.7 MHz and 37.9 MHz. In this case, the vibration meter operates on a 50 MHz measurement channel with a decimation ratio of 1. When the vibration velocity of the workpiece is between 3 m / s and 7.5 m / s, the corresponding Doppler frequency shift is between 9.5 MHz and 23.7 MHz. In this case, the vibration meter operates on a 25 MHz measurement channel with a decimation ratio of 1. When the vibration velocity of the workpiece is between 1 m / s and 3 m / s, the corresponding Doppler frequency shift is... When the Doppler frequency shift is between 3.16MHz and 9.5MHz, the vibration meter operates on a 10MHz measurement channel with a decimation factor of 2. When the vibration velocity of the workpiece is between 0.1m / s and 1m / s, the corresponding Doppler frequency shift is between 0.32MHz and 3.16MHz. In this case, the vibration meter operates on a 5MHz measurement channel with a decimation factor of 4. When the vibration velocity of the workpiece is less than 0.1m / s, the corresponding Doppler frequency shift is less than 0.32MHz. In this case, the vibration meter operates on a 1MHz measurement channel with a decimation factor of 6, which is used to measure low vibration velocities. In actual vibration velocity measurement, the vibration velocity measurement channel needs to be determined experimentally, including the down-conversion setting and decimation factor of the analog multiplier. For example, when the vibration velocity is uncertain, to meet the measurement range, the velocity measurement setting is adjusted to the high-speed setting. This means that the multiplier is connected to a unit step signal, and the signal is not down-converted. The signal is then acquired by a high-speed ADC and multiplied with an orthogonal signal before being connected to a channel with a decimation factor of 1, ensuring that the signal does not aliased. After measuring the vibration velocity using the high-speed setting, the setting is adjusted according to the actual vibration velocity of the object being measured, switching to a suitable measurement channel. At the same time, the decimation factor is increased while ensuring that the signal does not aliased. The measurement channel composed of the decimation filter and the down-conversion of the analog multiplier can reduce the computational load of the subsequent FPGA calculation of the arctangent without causing information loss, ensuring the real-time performance of the vibration velocity measurement.

[0082] The signal after the decimation filter is represented as follows:

[0083]

[0084]

[0085] The vibration displacement of the measured object can then be obtained by arctangent demodulation:

[0086]

[0087] The velocity of the measured object can be obtained by taking the derivative, i.e.:

[0088]

[0089] For digital demodulation systems, all values ​​are discrete; therefore, the vibration velocity demodulation formula can be rewritten as:

[0090]

[0091] In the formula, dt is the time corresponding to the sampling rate (or the time corresponding to the downsampling rate), from which the vibration velocity can be obtained, and the frequency discrimination curve can be plotted at the same time.

[0092] For high-speed digital demodulation systems, solving the arctangent of trigonometric functions is computationally very resource-intensive, while using lookup tables requires a large amount of memory. For FPGAs with limited ROM resources, these solutions are impractical. Therefore, the CORDIC algorithm is used to solve the arctangent, ensuring accuracy while meeting the system's high-speed demodulation requirements.

[0093] Finding the arctangent is equivalent to finding the corresponding angle θ. Assuming the initial input is (x1, y1), the CORDIC vector pattern continuously rotates the vector to make the initial value y1 approach 0. The angle accumulated during this process is the arctangent value we are looking for.

[0094] The method to rotate point (x2, y2) by an angle θ to point (x1, y1) in the xy coordinate plane is as follows:

[0095]

[0096] Introducing the factor cosθ, the equation can be written in the following form:

[0097]

[0098] If we remove the factor cosθ, we obtain the pseudo-rotation equation:

[0099]

[0100] After rotation, the angle of rotation is θ, but the magnitude is incorrect. However, the values ​​of x and y have increased to their original values. The CORDIC algorithm uses the pseudo-rotation method described above for approximation, and the rotation angle at each step satisfies:

[0101] tanθ i =2 -i

[0102] At this point, the iterative equation becomes:

[0103]

[0104] The complex trigonometric function solution is transformed into shift operations and addition / subtraction operations. The above describes the iterative change of one rotation. Throughout the iteration process, the iterative angle needs to be continuously accumulated. An initial value z0 is set as the desired rotation angle, and iteratively iterates until it approaches 0. During the rotation, to ensure accuracy and make z as close to 0 as possible, d is used. i To determine the direction of rotation, it is represented as d. i =-sign(x i y i ).

[0105] Then the decision operator d is added. i The iterative equation is expressed as:

[0106]

[0107] Therefore, each iteration requires two shift operations and one lookup table operation (to look up θ). i The derivation above ignores the factor cosθ, which increases its magnitude. Multiplying by this factor in each iteration would greatly increase computational resources. However, as the number of iterations increases, it can be observed that after multiplying all factors 13 times, they tend to a constant value, i.e. If the number of calculations is small, the scaling factor can be pre-calculated and multiplied after each iteration. Alternatively, to ensure the accuracy of the solved angle remains within a certain range, a threshold can be set, and iteration can stop only when the accuracy reaches the threshold range.

[0108] The final iteration result is as follows:

[0109]

[0110] Where z0 is the initial value set, usually 0, and after iteration z n This is the arctangent value we are looking for. The initial values ​​x2 and y2 are the corresponding Q-channel and I-channel signals.

[0111] By obtaining the frequency discrimination characteristic curve and comparing it with the actual vibration velocity, it can be found that the frequency discrimination voltage is proportional to the original carrier signal. However, when the vibration velocity is relatively large and the frequency deviation is relatively large, the demodulated waveform will be distorted. It is necessary to run the subsequent interpolation program based on the frequency discrimination characteristic curve to obtain the accurate vibration velocity. The obtained filter estimate is interpolated and transformed using the interpolation calculation algorithm to obtain the corresponding vibration velocity.

[0112] When interpolating the frequency discrimination characteristic curve, the curve is first divided into 16 intervals. Each interval corresponds to the smaller demodulated vibration velocity and the actual vibration velocity, and there is a corresponding interpolation formula. The filtered frequency discrimination voltage value is calculated as the final demodulation velocity. The corresponding interpolation formula is selected, and the demodulated vibration velocity is substituted into the interpolation formula to calculate the actual vibration velocity, thus completing the measurement of the vibration velocity.

[0113] The actual vibration velocity is -12 m / s to 12 m / s, and the corresponding demodulated vibration velocity is -10.26 m / s to 10.26 m / s. These are divided into 16 intervals for interpolation using a polynomial fitting method. Here, y represents the actual vibration velocity, and x represents the demodulated vibration velocity. This method ensures that the sum of squared residuals of the interpolation curves is minimized, and the correlation coefficient between the actual and demodulated vibration velocity curves after interpolation is greater than 0.998. Piecewise interpolation reduces errors. Furthermore, the interpolation function in each small interval depends only on the node values ​​of that segment. Therefore, each node only affects its neighbors, preventing the data error from expanding during calculation and ensuring the stability of the interpolation process. The interval divisions and corresponding interpolation formulas are shown in Table 1.

[0114] Table 1 Vibration Velocity Interpolation Table

[0115]

[0116]

[0117] As a preferred embodiment, the vibration velocity demodulation method of the present invention includes the following steps:

[0118] (4) By selecting the two switches, the unit step signal and the first measurement channel are connected without down-conversion to obtain the initial vibration velocity value.

[0119] (5) Based on the initial vibration velocity value, vibration velocity measurements are performed in the following different situations:

[0120] 1) When the initial vibration velocity value is between 7.5m / s and 12m / s, make the vibration meter work in the first measurement channel and set the sampling ratio to 1;

[0121] 2) When the initial vibration velocity value is between 3m / s and 7.5m / s, the vibration meter operates in the 25MHz measurement channel and the sampling factor is set to 1.

[0122] 3) When the initial vibration velocity value is between 1m / s and 3m / s, the vibration meter operates in the 10MHz measurement channel and the sampling rate is set to 2.

[0123] 4) When the initial vibration velocity value is between 0.1m / s and 1m / s, the vibration meter operates in the 5MHz measurement channel and the sampling rate is set to 4.

[0124] 5) When the initial vibration velocity value is less than 0.1 m / s, the vibration meter is working in the 1MHz measurement channel and the sampling rate is set to 6.

[0125] 6) Demodulate and obtain the vibration velocity;

[0126] (6) If the vibration meter is working in the 1MHz measurement channel and the measured vibration velocity is less than 0.02m / s, switch the probe to the laser ranging mode and use the laser ranging module to measure the vibration displacement of the object under test within a fixed time interval to directly obtain the vibration velocity corresponding to the extremely low vibration frequency.

[0127] Specifically, the method for demodulating and obtaining the vibration velocity is as follows:

[0128] 1) The high-speed ADC circuit acquires the signal after down-conversion by the analog multiplier at a sampling rate of 250Msps;

[0129] 2) The numerically controlled oscillator (NCO) generates two orthogonal signals, the frequency of which is the difference between the center frequency of the acousto-optic frequency shifter and the down-conversion frequency of the analog multiplier.

[0130] 3) The signal acquired by the high-speed ADC circuit is multiplied with the quadrature signal generated by the NCO, and after low-pass filtering, the I and Q signals are obtained. At this time, the frequencies of the two signals are down-converted by the multiplier and down-converted by the demodulation, and only contain frequency offset information, which is in an oversampling state.

[0131] 4) The CIC decimation filter and HB decimation filter are cascaded to match the previous analog multiplier down-conversion measurement channel for different vibration velocities.

[0132] 5) Divide the extracted I and Q signals, and then use the CORDIC algorithm to perform arctangent calculation to obtain the instantaneous displacement of the measured object;

[0133] 6) Obtain the vibration velocity by differentiating the displacement, and simultaneously plot the frequency discrimination characteristic curve;

[0134] 7) Divide the filtered frequency discrimination curve into 16 intervals. Each interval corresponds to the smaller demodulated vibration velocity and the actual vibration velocity, and there is a corresponding interpolation formula. Calculate the filtered frequency discrimination voltage value into the final demodulation speed, select the corresponding interpolation formula, substitute the demodulated vibration velocity into the interpolation formula to calculate the actual vibration velocity, and complete the measurement of vibration velocity.

Claims

1. A vibration velocity demodulation system for a laser Doppler vibrometer, wherein the laser Doppler vibrometer includes two modes: laser vibration measurement and laser ranging. In the laser vibration measurement mode, the probe converts the acquired frequency-shifted laser interference signal into a beat frequency electrical signal and outputs it. The system is characterized in that... It includes a high-precision heterodyne conditioning circuit, a down-conversion module with an analog multiplier, a high-speed ADC circuit, and a high-speed acquisition and demodulation module, among which... The high-precision heterodyne conditioning circuit takes a beat frequency electrical signal converted from a frequency-shifted laser interference signal as its input, and passes through a high-pass filter and amplification circuit, a limiting circuit, and a low-pass filter circuit in sequence. The output is a processed intermediate frequency signal. The downconversion module includes an analog multiplier, a first switch, a second switch, and measurement channels for different downconversion frequencies suitable for low and medium vibration velocities less than 7.5 m / s. The first input terminal of the analog multiplier is connected to the intermediate frequency signal processed by a high-precision heterodyne conditioning circuit, and the other end is connected to the second switch. The FPGA is used to generate multiple square waves of different frequencies and one unit step signal to form a measurement channel with different down-conversion frequencies suitable for medium and low vibration velocities. The multiple square waves are processed by their respective bandpass filters to obtain sine waves of different frequencies. The unit step signal generated by the FPGA or the sine waves of different frequencies processed by the bandpass filter can be selectively connected to the second input terminal of the analog multiplier through the first switch. The output of the analog multiplier can be selectively connected to the first measurement channel via a second switch, or connected to a measurement channel with a measurement frequency lower than that of the first measurement channel after passing through a corresponding four-way low-pass filter. The output of each measurement channel is sequentially passed through a high-speed ADC circuit and a high-speed acquisition and demodulation module to obtain the vibration velocity.

2. The vibration velocity demodulation system of the laser Doppler vibrometer according to claim 1, characterized in that, The first measurement channel is a 50MHz measurement channel.

3. The vibration velocity demodulation system of the laser Doppler vibrometer according to claim 1, characterized in that, The square waves are four-channel, generated by the internal crystal oscillator of the FPGA with frequencies of 49MHz, 45MHz, 40MHz and 25MHz respectively; the measurement channels of different frequencies include a 25MHz measurement channel, a 10MHz measurement channel, a 5MHz measurement channel and a 1MHz measurement channel.

4. A vibration velocity demodulation method implemented using the vibration velocity demodulation system according to any one of claims 1-3, characterized in that, Includes the following steps: (1) By selecting the two switches, the unit step signal and the first measurement channel are connected without down-conversion to obtain the initial vibration velocity value; (2) Based on the initial vibration velocity value, vibration velocity measurements are performed in the following different situations: When the initial vibration velocity value is between 7.5m / s and 12m / s, the vibration meter is set to work in the first measurement channel with a sampling rate of 1. When the initial vibration velocity value is between 3m / s and 7.5m / s, the vibration meter operates in the 25MHz measurement channel and the sampling rate is set to 1. When the initial vibration velocity value is between 1m / s and 3m / s, the vibration meter operates in the 10MHz measurement channel and the sampling rate is set to 2. When the initial vibration velocity value is between 0.1m / s and 1m / s, the vibration meter operates in the 5MHz measurement channel and the sampling rate is set to 4. When the initial vibration velocity value is less than 0.1 m / s, the vibration meter is working in the 1MHz measurement channel and the sampling rate is set to 6. Demodulate and obtain the vibration velocity; (3) If the vibration meter is working in the 1MHz measurement channel and the measured vibration velocity is less than 0.02m / s, use the optical switch to switch the probe to the laser ranging mode, use the laser ranging module to measure the vibration displacement of the measured object within a fixed time interval, and directly obtain the vibration velocity corresponding to the extremely low vibration frequency.

5. The vibration velocity demodulation method according to claim 4, characterized in that, The method for demodulating and obtaining vibration velocity is as follows: The high-speed ADC circuit acquires the signal after down-conversion by the analog multiplier at a sampling frequency of 250Msps. The numerically controlled oscillator (NCO) generates two orthogonal signals, the frequency of which is the difference between the center frequency of the acousto-optic frequency shifter and the down-conversion frequency of the analog multiplier. The signal acquired by the high-speed ADC circuit is multiplied with the quadrature signal generated by the NCO, and after low-pass filtering, I and Q signals are obtained. At this time, the frequencies of the two signals are down-converted by the analog multiplier and demodulated down-converted, and only contain frequency offset information, which is in an oversampled state. A cascaded CIC decimation filter and an HB decimation filter are used to match the previous analog multiplier down-conversion measurement channel for different vibration velocities. The extracted I and Q signals are divided, and then the arctangent operation is performed using the CORDIC algorithm to obtain the instantaneous displacement of the measured object. The vibration velocity is obtained by differentiating the displacement, and the frequency discrimination characteristic curve is plotted at the same time. The filtered frequency discrimination curve is divided into 16 intervals, each interval corresponding to the smaller demodulated vibration velocity and the actual vibration velocity, and there is a corresponding interpolation formula. The filtered frequency discrimination voltage value is calculated as the final demodulation speed. The corresponding interpolation formula is selected, and the demodulated vibration velocity is substituted into the interpolation formula to calculate the actual vibration velocity, thus completing the measurement of vibration velocity.

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