A demodulation system and method for simultaneously demodulating displacement and distance in real time

Abstract

The application provides a demodulation system and method for simultaneously demodulating displacement and distance in real time, belongs to the field of optical fiber interferometer measurement, and comprises a collection unit for collecting displacement interference signals, distance main interference signals and distance auxiliary interference signals, and a processing unit connected with the collection unit; the processing unit comprises a PL end and a PS end; the PL end is provided with parallel displacement preprocessing modules and distance preprocessing modules; the PS end is provided with parallel displacement calculation modules and distance calculation modules; the displacement preprocessing modules are bidirectionally connected with the displacement calculation modules; and the distance calculation modules are connected with the distance preprocessing modules. The application realizes high-precision measurement functions of high-speed real-time relative displacement and absolute distance in the same system.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical fiber interferometer measurement, and particularly relates to a demodulation system and method for simultaneously and real-time demodulating displacement and distance. BACKGROUND

[0002] An optical fiber sensor is a sensor device that uses an optical fiber as a transmission medium to sense external physical changes and thus changes the optical signal in the optical fiber. It is widely used in optical precision measurement of displacement, vibration and other high-resolution requirements. With the development of science and technology and the progress of precision equipment, precise position and topography measurement is increasingly required in the manufacture and assembly of precision equipment, such as the structure size of the nozzle of a space engine, the servo valve seat, the turbine pump impeller blade tip angle, the structure size and assembly gap measurement of the blade disc throat and the drum cavity of an aero-engine. Therefore, it is of great significance to break through the technical bottleneck of the optical fiber F-P interference sensor that cannot simultaneously and high-speed real-time demodulate displacement / distance, and to realize high-precision simultaneous measurement of nanoscale relative displacement change and micrometer-scale absolute distance value.

[0003] The phase generated carrier (PGC) algorithm is a relatively classic relative displacement measurement method. Early PGC demodulation algorithms are completed on a PC host. With the development of signal processing hardware, digital signal processing based on FPGA and DSP is a common demodulation method. ZYNQ series FPGA is used by the University of Defense Technology of the Chinese People's Liberation Army to demodulate an optical fiber vector hydrophone (CN108519146B), to improve the sampling rate and system integration; Changsha Haidun Technology Co., Ltd. uses an FPGA processor to measure the sound pressure sensitivity of a fiber hydrophone (CN205808543U). In terms of absolute distance measurement, the sweep frequency nonlinear correction ranging method based on similar triangle interpolation sampling (CN112946611A) and the sweep frequency interference ranging signal processing method (CN113253241A) proposed by Liu Guodong's research group of Harbin Institute of Technology. Weng Jidong et al. proposed a full-optical fiber frequency domain interference absolute distance measurement method and device (CN104197844B). However, traditional optical fiber sensors can usually only measure relative displacement or absolute distance, and cannot simultaneously measure high-speed real-time relative displacement and absolute distance, which is difficult to meet the needs of precision part manufacturing and measurement in the field of aerospace. SUMMARY

[0004] The purpose of the present application is to provide a demodulation system and method for simultaneously and real-time demodulating displacement and distance, to realize high-precision measurement of high-speed real-time relative displacement and absolute distance in the same system.

[0005] The present application is implemented by the following technical solutions:

[0006] The application discloses a demodulation system for simultaneously demodulating displacement and distance in real time, which comprises a collecting unit for collecting displacement interference signals, distance main interference signals and distance auxiliary interference signals, and a processing unit connected with the collecting unit, wherein the processing unit comprises a PL end and a PS end, the PL end is provided with parallel displacement preprocessing modules and distance preprocessing modules, the PS end is provided with parallel displacement calculation modules and distance calculation modules, the displacement preprocessing modules are bidirectionally connected with the displacement calculation modules, the displacement preprocessing modules firstly perform downsampling on the collected displacement interference signals, then mix the downsampled displacement interference signals with carrier base frequency signals and transmit the obtained data to the displacement calculation modules, the displacement calculation modules calculate base frequency phase compensation amounts according to the received data and feed back to the displacement preprocessing modules, the displacement preprocessing modules make the carrier base frequency signals in phase synchronization according to the base frequency phase compensation amounts, then mix the downsampled displacement interference signals with carrier double frequency signals and transmit the obtained data to the displacement calculation modules, the displacement calculation modules calculate double frequency phase compensation amounts according to the received data and feed back to the displacement preprocessing modules, the displacement preprocessing modules make the carrier double frequency signals in phase synchronization according to the double frequency phase compensation amounts, then mix the downsampled displacement interference signals with the in-synchronization carrier base frequency signals and carrier double frequency signals respectively and transmit the obtained data to the displacement calculation modules, and the displacement calculation modules obtain relative displacement values according to the received data, the distance calculation modules are connected with the distance preprocessing modules, the distance preprocessing modules perform downsampling on the collected distance main interference signals and distance auxiliary interference signals and transmit the obtained data to the distance calculation modules, and the distance calculation modules calculate absolute distance information according to the received data.

[0007] Further, the displacement preprocessing modules mix the downsampled displacement interference signals with sine components and cosine components of the carrier base frequency signals respectively to obtain base frequency sine mixed signals I x '(t) and base frequency cosine mixed signals I y '(t), the displacement calculation modules divide the base frequency sine mixed signals by the base frequency cosine mixed signals and obtain the base frequency phase compensation amounts through arctangent calculation, the displacement preprocessing modules mix the downsampled displacement interference signals with sine components and cosine components of the carrier double frequency signals respectively to obtain double frequency sine mixed signals I x "(t) and double frequency cosine mixed signals I y "(t), the displacement calculation modules divide the double frequency sine mixed signals by the double frequency cosine signals and obtain the double frequency phase compensation amounts through arctangent calculation, and the displacement preprocessing modules mix the downsampled displacement interference signals with the in-synchronization carrier base frequency signals and carrier double frequency signals respectively to obtain sine components I x (t) and cosine components Iy(t).

[0008] Furthermore, the displacement calculation module includes a correction module for amplitude and phase correction, which performs corrections according to the formulas... and to I x (t) and I y (t) is corrected, where max(I) x (t)) and min(I x (t) represents the sinusoidal component I. x The maximum and minimum values ​​of (t), max(I y (t)) and min(I y (t) represents the cosine component I. y The maximum and minimum values ​​of (t).

[0009] Furthermore, the displacement calculation module also includes a displacement phase calculation module, a displacement phase unwrapping module, and a displacement result calculation module connected in sequence. The displacement phase calculation module calculates the corrected sinusoidal components. Sum and cosine components The phase data is obtained by dividing the phase data and performing an arctangent operation on the result. The phase displacement unwrapping module is used to determine whether the difference between two adjacent phases in the phase data exceeds the range of [-π, π]. If so, the next phase value is added to or subtracted from an integer multiple of 2π until the obtained phase data is continuous. The displacement result calculation module is used to calculate the relative displacement value based on the carrier wavelength, free space refractive index and the result of the phase displacement unwrapping module.

[0010] Furthermore, the processing unit also includes a displacement transmission module connected between the displacement preprocessing module and the displacement calculation module, and a distance transmission module connected between the distance preprocessing module and the distance calculation module. The displacement transmission module is used to transmit the fundamental frequency sinusoidal mixing signal I... x ′(t) and the fundamental frequency cosine mixing signal I y ′(t), second harmonic sinusoidal mixer signal I x "(t) and the second harmonic cosine mixing signal I y "(t) Write to the FIFO. After the FIFO stores M1 data points, send the displacement interrupt signal to the displacement calculation module. The displacement calculation module reads the data from the FIFO at a clock frequency of H1 and calculates the sinusoidal component I." x After obtaining the cosine component Iy(t) and the sine component Iy(t), the sine component Iy(t) will be obtained. x(t) and cosine component Iy(t) are written into FIFO, when FIFO stores M2 data points, displacement interrupt signal is sent to displacement calculation module, displacement calculation module reads data from FIFO at H2 clock frequency; distance transmission module is used for writing data output by distance preprocessing module into RAM, after all data are written, M3 data points are read from RAM and written into FIFO every T1 period, and distance interrupt signal is sent to distance processing module, distance processing module reads data from FIFO at H3 clock frequency.

[0011] Further, the acquisition unit includes a first pre-amplification circuit and a first ADC module connected to each other, a second pre-amplification circuit and a second ADC module connected to each other, and a third pre-amplification circuit and a third ADC module connected to each other. The first pre-amplification circuit input end is connected to the displacement measuring interferometer, the second pre-amplification circuit input end is connected to the distance measuring main interferometer, and the third pre-amplification circuit input end is connected to the distance measuring auxiliary interferometer. The first ADC module output end, the second ADC module output end, and the third ADC module output end are respectively connected to the processing unit input end.

[0012] Further, the distance calculation module includes a fast peak searching module, an EMD module, a distance phase calculation module, a distance phase unwrapping module, a non-uniform fitting module, and a distance result calculation module connected in sequence. The fast peak searching module is used to output the peak position information of the down-sampled distance main interferometric signal and the distance auxiliary interferometric signal. The EMD module eliminates the direct current and envelope fluctuation of the corresponding signal according to the received peak position information. The distance phase calculation module performs arcsin operation on the data processed by the EMD module to extract the signal phase. The distance phase unwrapping module is used to take the inverse of the even interval in the data output by the distance phase calculation module according to the index of the peak contained in the peak position information, to realize the conversion from a triangular wave to a sawtooth wave, to realize phase unwrapping, to obtain the phase of the distance main interferometric signal and the phase of the distance auxiliary interferometric signal. The non-uniform fitting module performs phase domain non-uniform fitting on the phase of the distance main interferometric signal relative to the phase of the distance auxiliary interferometric signal by using the least square method. The distance result calculation module obtains absolute distance information according to the fitting result, the optical path of the distance auxiliary interferometer, the refractive index of the measurement space, and the fiber refractive index.

[0013] Further, the acquisition unit and the processing unit are implemented based on SoC, the chip used by the SoC is Zynq UltraScale+MPSoC, the displacement calculation module is implemented by the displacement core of the chip, and the distance calculation module is implemented by the distance core of the chip. The displacement core and the distance core are two ARM Cortex-A53 processor cores in the chip Zynq UltraScale+MPSoC, which process data in parallel.

[0014] The application also realizes the above-mentioned technical effects through the following technical solutions.

[0015] The demodulation method based on the demodulation system for simultaneously demodulating displacement and distance in real time according to any one of the above has the following steps.

[0016] Step S1, the acquisition unit acquires the displacement interference signal, the distance main interference signal and the distance auxiliary interference signal respectively, and transmits the acquired signals to the processing unit.

[0017] Step S2, the processing unit processes the displacement data and the distance data in parallel: the displacement preprocessing module first performs downsampling on the acquired displacement interference signal, then mixes the downsampled displacement interference signal with the carrier base frequency signal and transmits the obtained data to the displacement calculation module, the displacement calculation module calculates the base frequency phase compensation amount according to the received data and feeds back to the displacement preprocessing module, the displacement preprocessing module synchronizes the phase of the carrier base frequency signal according to the base frequency phase compensation amount, then mixes the downsampled displacement interference signal with the carrier double frequency signal and transmits the obtained data to the displacement calculation module, the displacement calculation module calculates the double frequency phase compensation amount according to the received data and feeds back to the displacement preprocessing module, the displacement preprocessing module synchronizes the phase of the carrier double frequency signal according to the double frequency phase compensation amount, then mixes the downsampled displacement interference signal with the synchronized carrier base frequency signal and carrier double frequency signal respectively and transmits the obtained data to the displacement calculation module, and the displacement calculation module obtains the relative displacement value according to the received data; the distance calculation module is connected with the distance preprocessing module, the distance preprocessing module performs downsampling on the acquired distance main interference signal and distance auxiliary interference signal and transmits the obtained data to the distance calculation module, and the distance calculation module calculates the absolute distance information according to the received data.

[0018] Further, the acquisition unit and the processing unit are realized based on SoC, the chip used by the SoC is Zynq UltraScale+MPSoC ZU7EV, the displacement calculation module is realized by the displacement core of the chip, the distance calculation module is realized by the distance core of the chip, and the displacement core and the distance core are two ARM Cortex-A53 processor cores in the chip Zynq UltraScale+MPSoC, which process data in parallel.

[0019] The application has the following beneficial effects:

[0020] 1、The collection unit of the application collects displacement interference signals, distance main interference signals and distance auxiliary interference signals, the processing unit includes a PL end and a PS end, the PL end has parallel displacement preprocessing modules and distance preprocessing modules, the PS end has parallel displacement calculation modules and distance calculation modules, the advantages of the PL and the PS are combined, the PL processing parallel data streams and the PS general software execution capability are fully utilized, the demodulation system has the ability of simultaneous real-time demodulation of displacement and distance, has high efficiency, flexibility and universality, and provides a new solution for the manufacturing and measurement of precision parts equipment; under the premise of ensuring the accuracy and reliability of the measurement results of the system, the data is down-sampled, the delay of data transmission and the data processing amount are reduced, the processing time of the PS end is reduced as a whole, and therefore the running speed of the system and the overall efficiency of the system are improved.

[0021] 2、When the relative displacement measurement is performed, the two signals filtered by mixing are corrected, the periodic nonlinear error caused by the unequal amplitude error for the measurement result is eliminated, and the precision of displacement measurement is effectively improved.

[0022] 3、When the absolute distance measurement is performed, the sweep frequency nonlinear error is eliminated through non-uniform fitting, the uncertainty of the demodulation result caused by factors such as unstable light source is avoided, the demodulation result has high precision and high resolution, and the accuracy of the demodulation result of the system and the stability of the demodulation system are improved. BRIEF DESCRIPTION OF DRAWINGS

[0023] The application will be further described in detail below with reference to the drawings.

[0024] Figure 1 It is a principle block diagram of the demodulation system of the application.

[0025] Figure 2 It is an F-P cavity interference displacement measurement optical path based on sinusoidal phase modulation.

[0026] Figure 3 It is a principle block diagram of the carrier base frequency signal phase delay synchronization module of the application.

[0027] Figure 4 It is a principle block diagram of the carrier double frequency signal phase delay synchronization module of the application.

[0028] Figure 5 It is an F-P cavity interference distance measurement optical path based on linear frequency modulation.

[0029] Figure 6 It is a host computer display interface for displacement measurement of the application.

[0030] Figure 7 It is a result graph for distance measurement of the application. DETAILED DESCRIPTION

[0031] As Figure 1 shown, the demodulation system for simultaneously demodulating displacement and distance in real time comprises a collection unit for collecting displacement interference signals, distance main interference signals and distance auxiliary interference signals, a processing unit connected with the collection unit, and an upper computer bidirectionally connected with the processing unit. The collection unit comprises a first pre-amplification circuit and a first ADC module connected with each other, a second pre-amplification circuit and a second ADC module connected with each other, and a third pre-amplification circuit and a third ADC module connected with each other. The output end of the displacement measurement interferometer is connected with the input end of the first pre-amplification circuit through a photoelectric conversion device. The output end of the distance measurement main interferometer is connected with the input end of the second pre-amplification circuit through a photoelectric conversion device. The output end of the distance measurement auxiliary interferometer is connected with the input end of the third pre-amplification circuit through a photoelectric conversion device. The output end of the first ADC module, the output end of the second ADC module, and the output end of the third ADC module are respectively connected with the input end of the processing unit. The processing unit comprises a PL (programmable logic) end and a PS end processing system. The PL end has a parallel displacement preprocessing module and a distance preprocessing module. The PS end has a parallel displacement calculation module and a distance calculation module. The displacement preprocessing module is bidirectionally connected with the displacement calculation module, and the distance calculation module is connected with the distance preprocessing module.

[0032] In this embodiment, the collection unit and the processing unit are realized based on SoC (system on chip). The chip used by the SoC is Zynq UltraScale+MPSoC ZU7EV. The chip has four Arm Cortex-A53, the core frequency of which can reach 1.5GHz. The parallel processing capacity of the four cores is utilized to allocate the cores to different tasks, i.e. the displacement calculation module is realized by the displacement core of the chip, the distance calculation module is realized by the distance core of the chip, the displacement core and the distance core process data in parallel, the TCP core of the chip realizes transmission with the upper computer, and the system realizes the integration of multiple functions such as calculation of relative displacement, calculation of absolute distance, and data communication, so that multiple tasks are completed on one chip, and the processing efficiency and real-time performance of the system are greatly improved. The PL end and the PS end communicate and exchange data through a high-speed bus (such as AXI bus) or other specific interfaces. The AD chip is AD9643, the sampling frequency in actual use is 250MHz, the clock chip is LMK04828, the communication interface uses gigabit Ethernet, and the system board is equipped with 4GB of DDR4 memory.

[0033] For the TCP core, the transmission process is stable. It receives parameter configuration instructions from the host computer, including parameters used by the distance and displacement cores, such as the wavelength of the displacement measurement interferometer, the thresholds of the main and auxiliary interferometers for distance measurement, correction parameters, and the percentage of redundant points removed. These parameters are stored in the allocated DDR memory space. Upon successful reception, it sends a reception success flag back to the host computer. When it receives instructions to transmit raw data and calculation results, the data is unpacked into 1024-byte packets. Each packet contains 1008 bytes of data, and the frame header, frame trailer, CRC checksum, content length, data type, and other parameters occupy 16 bytes. If the remaining transmission content is less than 1008 bytes, the remaining content (less than 1008 bytes) is immediately packetized separately with a 16-byte identifier and sent.

[0034] The principle of relative displacement measurement is as follows:

[0035] Under normal demodulation conditions: the optical path for FP-cavity interferometric displacement measurement based on linear modulation is as follows: Figure 2 As shown, the laser beam propagates in the optical path system and is incident on the reference surface in the fiber optic probe structure. Part of the light forms the reference beam and returns along the original optical path. The remaining light exits along the fiber optic probe, is incident on the surface of the target object, is reflected by the target object surface to form the measurement beam, and is coupled back to the original optical path to interfere with the reference beam.

[0036] The displacement interference signal can be expressed as I(t,τ)=A+Bcos[Ccos(ω m t)+ω0τ], where A and B are constants, ω0 is the center angular frequency, and ω m ω is the angular frequency of the modulated signal, C is the modulation depth, and τ is the time delay between the measured light and the reference light.

[0037] Displacement interference signal and carrier fundamental frequency signal cos(ω) m t) The sinusoidal component I of the displacement interference signal is obtained by mixing and filtering. x (t)=-BGJ1(C)sinω0τ,Displacement interference signal and carrier second harmonic signalcos(2ω m t) The cosine component I of the displacement interference signal is obtained by mixing and filtering. y (t)=-BGJ2(C)cosω0τ, where J1(C) is a first-order Bessel function and J2(C) is a second-order Bessel function;

[0038] The sine and cosine components, after amplitude-phase correction, are expressed as follows: and Dividing the two yields The modulation depth needs to be selected at a proper value so that J1(C) is equal to J2(C), and then an inverse tangent operation is performed to demodulate ω0τ, and then the delay time τ of the measurement light and the reference light is solved, and according to τ and the speed of light c, the displacement information of the target object can be solved by the formula Δx = τc.

[0039] However, in an actual hardware demodulation system, there are light propagation delay, conversion delay of ADC in the hardware circuit and photoelectric conversion delay, which will cause a phase difference between the carrier signal and the modulation carrier signal cos(ω m t) of the displacement interference signal That is, the displacement interference signal should be expressed as At this time, carrier phase delay synchronization needs to be performed, that is:

[0040] Considering the initial phase, the carrier fundamental frequency signal is expressed as The carrier double-frequency signal is expressed as

[0041] In order to compensate the phase difference of the carrier fundamental frequency A carrier fundamental frequency phase delay synchronization module as shown in Figure 3 is introduced, and the carrier fundamental frequency phase delay synchronization utilizes the quadrature phase-locked algorithm to calculate That is, the displacement interference signal is multiplied by the sine component and the cosine component of the carrier fundamental frequency signal, and after low-pass filtering, the fundamental frequency sine mixing signal and the fundamental frequency cosine mixing signal are obtained. The carrier fundamental frequency phase compensation amount is obtained by inverse tangent calculation Then, the carrier fundamental frequency phase delay is compensated by using so that the carrier fundamental frequency phase is automatically synchronized.

[0042] In order to compensate the phase difference of the carrier double frequency A carrier double frequency phase delay synchronization module as shown in Figure 4 is introduced, and the carrier double frequency phase delay synchronization utilizes the quadrature phase-locked algorithm to calculate That is, the displacement interference signal is multiplied by the sine component and the cosine component of the carrier double frequency signal, and after low-pass filtering, the double-frequency sine mixing signal and the double-frequency cosine mixing signal are obtained. The carrier double frequency phase compensation amount is obtained by inverse tangent calculation Then, the carrier double frequency phase delay is compensated by using so that the carrier double frequency phase is automatically synchronized.

[0043] For displacement demodulation, the system operates in three modes: carrier fundamental frequency phase delay synchronization mode, carrier second harmonic phase delay synchronization mode, and displacement demodulation mode. After power-on initialization, the displacement preprocessing module first performs four-stage downsampling on the acquired displacement interference signal, reducing the data rate from 250MHz to 100kHz. Downsampling is achieved using a CIC decimation filter. Then, it enters the carrier fundamental frequency phase delay synchronization mode. In this mode, the displacement preprocessing module mixes and filters the downsampled displacement interference signal with the sine and cosine components of the carrier fundamental frequency signal to obtain the fundamental frequency sinusoidal mixed signal I. x ′(t) and the fundamental frequency cosine mixing signal I y ′(t), and the fundamental frequency sinusoidal mixing signal I x ′(t) and the fundamental frequency cosine mixing signal I y The signal '(t) is transmitted to the displacement calculation module, which then calculates the fundamental frequency sinusoidal mixing signal I. x ′(t) and the fundamental frequency cosine mixing signal I y The fundamental frequency phase compensation is obtained by dividing by '(t) and calculating the arctangent. and the Feedback is sent to the displacement preprocessing module, which sets the phase control word of the DDS that generates the carrier fundamental frequency signal to... Phase synchronization is performed, followed by a carrier second-harmonic phase delay synchronization mode. In this mode, the displacement preprocessing module mixes and filters the downsampled displacement interference signal with the sine and cosine components of the carrier second-harmonic signal to obtain the second-harmonic sinusoidal mixed signal I. x "(t) and the second harmonic cosine mixing signal I y "(t) is transmitted to the displacement calculation module, which then converts the second harmonic sinusoidal mixed signal I into a frequency-adjusted signal. x "(t) and the second harmonic cosine signal I y The second harmonic phase compensation is obtained by dividing by ″(t) and calculating the arctangent. This information is then fed back to the displacement preprocessing module, which uses the second harmonic phase compensation amount as a reference. The phase of the carrier frequency second harmonic signal is synchronized, and then the displacement demodulation mode is entered. In this mode, the displacement calculation module mixes and filters the downsampled displacement interference signal with the synchronized carrier fundamental frequency signal and the carrier frequency second harmonic signal to obtain the sinusoidal component I. x (t) and cosine component Iy(t), and the sine component I x The cosine component Iy(t) and the cosine component Iy(t) are transmitted to the displacement calculation module, which then obtains the relative displacement value based on the received data.

[0044] More specifically, the displacement calculation module comprises a correction module, a displacement phase calculation module, a displacement phase unwrapping module and a displacement result calculation module connected in sequence. The correction module is used to perform amplitude and phase correction to eliminate the periodic nonlinear error brought by the unequal irradiation error to the measurement result, so as to improve the accuracy of displacement measurement. The correction module first detects the maximum and minimum values of the sine component I x (t) and the cosine component Iy(t), and then corrects I (t) and I (t) according to the formulas x (t) and I y (t) respectively, wherein max(I x (t)) and min(I x (t)) are the maximum and minimum values of the sine component I x (t) respectively, and max(I y (t)) and min(I y (t)) are the maximum and minimum values of the cosine component I y (t) respectively.

[0045] The displacement phase calculation module divides the corrected sine component I (t) by the corrected cosine component I (t), and performs an arctangent operation on the result of the division to obtain the phase data of the corrected data. The displacement phase unwrapping module is used to determine whether the difference between two adjacent phases in the phase data exceeds the range [-π, π]. If so, the latter phase value is added or subtracted by an integer multiple of 2π until the obtained phase data is continuous. The displacement result calculation module is used to calculate the relative displacement value according to the carrier wavelength, the free space refractive index and the result of the displacement phase unwrapping module.

[0046] Absolute distance measurement principle:

[0047] The basic principle of distance measurement is the frequency modulation continuous wave beat frequency interference principle. The frequency or phase of the beat frequency signal is analyzed to extract the required distance information. The specific measurement optical path is shown in Figure 5 .

[0048] The modulation signal generation module outputs a triangular wave to the light source modulator for modulating the light source. The frequency modulation slope α = Δω / T m , wherein α is the instantaneous frequency sweeping speed of the light source, Δω is the tuning range of the light source, and T m is the period of the modulation signal.

[0049] For the distance measurement main interferometer and the distance measurement auxiliary interferometer, the expressions of the detected interference light signals are and , wherein I m and I rFor the measured light intensity, I m1 , m2 and I r1 , r2 For the light intensity of the two interfering lights, ω m = ατ m , r = ατ r For the frequency of the measured beat signal, φ m0 = ω0τ m , r0 = ω0τ r For the initial phase of the measured beat signal, ω0the frequency corresponding to the center wavelength of the interference laser, τ m For the time delay of the two light beams caused by the known auxiliary interferometer arm length difference, τ r For the time delay of the interference light caused by the to-be-measured spatial distance.

[0050] After phase demodulation is performed on the measurement interferometer and the auxiliary interferometer respectively, time-varying terms can be obtained: φ r = ατ r t, φ m = ατ m t,

[0051] Using the phase comparison method and performing conversion, the corresponding distance information can be solved: Wherein, n is the refractive index of the optical fiber, L r is the known distance measurement auxiliary interferometer arm length difference, and L is the to-be-measured distance.

[0052] The distance calculation module is connected with the distance preprocessing module, the distance preprocessing module performs one-level downsampling on the collected distance main interference signal and distance auxiliary interference signal, reduces the data rate from 250MHz to 50MHz, and transmits the data obtained by the downsampling to the distance calculation module, and the distance calculation module calculates the absolute distance information according to the received data.

[0053] Specifically, the distance calculation module comprises, in sequence, a fast peak searching module, an EMD module, a distance phase calculation module, a distance phase unwrapping module, a non-uniform fitting module and a distance result calculation module. The fast peak searching module normalizes and scales the received down-sampled distance main interferometric signal and distance auxiliary interferometric signal respectively, and sends the normalized signals to a threshold detector. According to the peak width, peak position, peak spacing information and peak feature appearance law of the interferometric signals, the fast peak searching module outputs the peak position information of the distance main interferometric signal and the distance auxiliary interferometric signal. The EMD (Empirical Mode Decomposition) module processes the received peak position information, reads the peak amplitude according to the peak index, and interpolates the maximum value and the minimum value respectively to obtain the upper envelope line and the lower envelope line of the signal. The two envelope lines are used to scale the signal to eliminate the direct current and envelope fluctuation of the signal. The distance phase calculation module performs arcsin operation on the data processed by the EMD module to extract the signal phase. The distance phase unwrapping module is used to take the inverse of the even interval in the data output by the distance phase calculation module according to the index of the peak contained in the peak position information, to realize the conversion from a triangular wave to a sawtooth wave, so as to realize phase unwrapping and obtain the phase of the distance main interferometric signal and the phase of the distance auxiliary interferometric signal. The non-uniform fitting module uses the least square method to perform phase domain non-uniform fitting on the phase of the distance main interferometric signal relative to the phase of the distance auxiliary interferometric signal, to realize the correction of the non-linearity of the light source sweep frequency. The distance result calculation module obtains the absolute distance information according to the fitting result, the optical path of the distance auxiliary interferometer, the refractive index of the measurement space and the fiber refractive index.

[0054] The processing unit further comprises a displacement transmission module connected between the displacement preprocessing module and the displacement calculation module, and a distance transmission module connected between the distance preprocessing module and the distance calculation module. In the displacement base frequency phase delay synchronization mode and the carrier double frequency phase delay synchronization mode, the displacement transmission module is used to write the base frequency sine mixing signal I x '(t) and the base frequency cosine mixing signal I y '(t), the double frequency sine mixing signal I x ″(t) and the double frequency cosine mixing signal I y ″(t) into the FIFO. When the FIFO stores 5k data points, a displacement interrupt signal is sent to the displacement calculation module. The displacement calculation module reads out the data from the FIFO at a clock frequency of 250MHz. A total of 100k data points are sent for 20 times. In the displacement demodulation mode, after the sine component I x (t) and the cosine component Iy(t) are calculated, the sine component I x(t) and cosine component Iy(t) are written into FIFO, when FIFO stores 5k data points, displacement interrupt signal is sent to displacement calculation module, displacement calculation module reads out data from FIFO with 250MHz clock frequency, data is continuously sent. Distance transmission module writes 42000 points of data after two frames extraction into RAM, after all data is written, 2k data points are read from RAM and written into FIFO every 20ms, and distance interrupt signal is sent to distance processing module, distance processing module reads out data from FIFO with 250MHz clock frequency, a total of 21 times. After absolute distance data is obtained by distance calculation module through data processing and calculation, absolute distance calculation completion response signal is sent to distance preprocessing module, after distance preprocessing module receives the response signal, next distance data transmission is started.

[0055] Based on the above simultaneous real-time demodulation displacement and distance demodulation system, the demodulation method comprises the following steps:

[0056] Step S1, the acquisition unit respectively acquires displacement interference signals, distance main interference signals and distance auxiliary interference signals, and transmits the acquired signals to the processing unit;

[0057] Step S2, the processing unit processes displacement data and distance data in parallel: the displacement preprocessing module first down-samples the acquired displacement interference signals, then mixes the down-sampled displacement interference signals with the carrier base frequency signal and transmits the obtained data to the displacement calculation module, the displacement calculation module calculates the base frequency phase compensation amount according to the received data and feeds back to the displacement preprocessing module, the displacement preprocessing module synchronizes the phase of the carrier base frequency signal according to the base frequency phase compensation amount, then mixes the down-sampled displacement interference signals with the carrier double frequency signal and transmits the obtained data to the displacement calculation module, the displacement calculation module calculates the double frequency phase compensation amount according to the received data and feeds back to the displacement preprocessing module, the displacement preprocessing module synchronizes the phase of the carrier double frequency signal according to the double frequency phase compensation amount, then mixes the down-sampled displacement interference signals with the synchronized carrier base frequency signal and carrier double frequency signal respectively and transmits the obtained data to the displacement calculation module, the displacement calculation module obtains the relative displacement value according to the received data; the distance calculation module is connected with the distance preprocessing module, the distance preprocessing module down-samples the acquired distance main interference signals and distance auxiliary interference signals and transmits the obtained data to the distance calculation module, the distance calculation module calculates the absolute distance information according to the received data.

[0058] For displacement, the nano stage displacement mode is sinusoidal vibration, the set sinusoidal vibration peak-to-peak value is the nano stage displacement value, under the condition of peak-to-peak value 1000nm, the display interface of the host computer is as follows Figure 6The measurement results are shown in Table 1, and the accuracy (root mean square error) is 9.4 nanometers.

[0059] Table 1: Sinusoidal vibration peak-to-peak 1000 nm measurement results

[0060] Number of tests Displacement (nm) Displacement error (nm) 1. 1008 8 2. 1007 7 3. 1002 2 4. 1007 7 5. 1008 8 6. 1008 8 7. 1016 16 8. 1016 16 9. 1006 6 10. 1006 6

[0061] For the distance, the displacement table is moved in steps of 10 microns, and the distance measurement results are shown in Table 1. Figure 7 The accuracy (root mean square error) of the absolute distance measurement results is 0.46 microns, and the resolution is 0.75 microns.

[0062] The present application achieves high-precision results of displacement measurement accuracy better than 10 nanometers, distance measurement accuracy better than 1 micron, and resolution better than 1 micron, providing reliable measurement data, which helps to improve the quality and accuracy of equipment manufacturing and measurement.

[0063] The above is only a preferred embodiment of the present application, and therefore cannot limit the scope of the present application. Equivalent changes and modifications made in accordance with the scope of the present application and the content of the specification should still be within the scope of the present application.

Claims

1. A demodulation system for simultaneous real-time demodulation of displacement and distance, characterized by: The collection unit comprises a displacement interference signal, a distance main interference signal and a distance auxiliary interference signal, and a processing unit connected with the collection unit, the processing unit comprises a PL end and a PS end, the PL end has parallel displacement preprocessing modules and distance preprocessing modules, the PS end has parallel displacement calculation modules and distance calculation modules, the displacement preprocessing modules are bidirectionally connected with the displacement calculation modules, the displacement preprocessing modules firstly perform downsampling on the collected displacement interference signal, then mix the downsampled displacement interference signal with a carrier base frequency signal and transmit the obtained data to the displacement calculation modules, the displacement calculation modules calculate base frequency phase compensation amounts according to the received data and feed back to the displacement preprocessing modules, the displacement preprocessing modules make the carrier base frequency signal phase synchronous according to the base frequency phase compensation amounts, then mix the downsampled displacement interference signal with a carrier double frequency signal and transmit the obtained data to the displacement calculation modules, the displacement calculation modules calculate double frequency phase compensation amounts according to the received data and feed back to the displacement preprocessing modules, the displacement preprocessing modules make the carrier double frequency signal phase synchronous according to the double frequency phase compensation amounts, then mix the downsampled displacement interference signal with the synchronous carrier base frequency signal and the synchronous carrier double frequency signal respectively and transmit the obtained data to the displacement calculation modules, the displacement calculation modules obtain relative displacement values according to the received data, the distance calculation modules are connected with the distance preprocessing modules, the distance preprocessing modules perform downsampling on the collected distance main interference signal and distance auxiliary interference signal and transmit the obtained data to the distance calculation modules, and the distance calculation modules calculate absolute distance information according to the received data.

2. The demodulation system for simultaneous real-time demodulation of displacement and distance of claim 1, wherein: The displacement preprocessing module mixes the down-sampled displacement interference signal with the sine component and the cosine component of the carrier base frequency signal respectively to obtain base frequency sine mixed signal I x ′(t) and base frequency cosine mixed signal I y ′(t), the displacement calculation module divides the base frequency sine mixed signal by the base frequency cosine mixed signal and obtains the base frequency phase compensation quantity through arctangent calculation, the displacement preprocessing module mixes the down-sampled displacement interference signal with the sine component and the cosine component of the carrier double frequency signal respectively to obtain double frequency sine mixed signal I x ″(t) and double frequency cosine mixed signal I y ″(t), the displacement calculation module divides the double frequency sine mixed signal by the double frequency cosine signal and obtains the double frequency phase compensation quantity through arctangent calculation, and the displacement preprocessing module mixes the down-sampled displacement interference signal with the synchronized carrier base frequency signal and the carrier double frequency signal to obtain sine component I x (t) and cosine component Iy(t).

3. A demodulation system for simultaneous real-time demodulation of displacement and distance according to claim 2, characterized in that: The displacement calculation module comprises a correction module for performing amplitude and phase correction, the correction module respectively correcting I and I x (t) and I y (t) according to the formulae max(I x (t)) and min(I x (t)) are the maximum and minimum values of the sine component I x (t), and max(I y (t)) and min(I y (t)) are the maximum and minimum values of the cosine component I y (t), respectively.

4. The demodulation system of claim 3, wherein: The displacement calculation module further comprises a displacement phase solving module, a displacement phase unwrapping module and a displacement result calculation module connected in sequence. The displacement phase solving module divides the corrected sine component and the cosine component , and performs an inverse tangent operation on the result of the division to obtain phase data of the corrected data. The displacement phase unwrapping module is used to determine whether the difference between two adjacent phases in the phase data exceeds the range of [-π, π]. If so, the next phase value is added or subtracted by an integer multiple of 2π until the obtained phase data is continuous. The displacement result calculation module is used to calculate a relative displacement value according to the wavelength of the carrier, the refractive index of free space and the result of the displacement phase unwrapping module.

5. A demodulation system for simultaneous real-time demodulation of displacement and distance according to claim 2 or 3 or 4, characterized in that: The processing unit also includes a displacement transmission module connected between the displacement preprocessing module and the displacement calculation module, and a distance transmission module connected between the distance preprocessing module and the distance calculation module. The displacement transmission module is used to transmit the fundamental frequency sinusoidal mixing signal I... x ′(t) and the fundamental frequency cosine mixing signal I y ′(t), second harmonic sinusoidal mixer signal I x "(t) and the second harmonic cosine mixing signal I y "(t) Write to the FIFO. After the FIFO stores M1 data points, send the displacement interrupt signal to the displacement calculation module. The displacement calculation module reads the data from the FIFO at a clock frequency of H1 and calculates the sinusoidal component I." x After obtaining the cosine component Iy(t) and the sine component Iy(t), the sine component Iy(t) will be obtained. x The cosine component Iy(t) and cosine component Iy(t) are written into the FIFO. After the FIFO stores M2 data points, the displacement interrupt signal is sent to the displacement calculation module. The displacement calculation module reads the data from the FIFO at a clock frequency of H2. The distance transmission module is used to write the data output by the distance preprocessing module into the RAM. After all the data is written, M3 data points are read from the RAM and written into the FIFO each time with a period of T1, and a distance interrupt signal is sent to the distance processing module. The distance processing module reads the data from the FIFO at a clock frequency of H3.

6. A demodulation system for simultaneous real-time demodulation of displacement and distance according to claim 1 or 2 or 3 or 4, characterized in that: The collection unit comprises a displacement interference signal, a distance main interference signal and a distance auxiliary interference signal, and a processing unit connected with the collection unit, the processing unit comprises a PL end and a PS end, the PL end has parallel displacement preprocessing modules and distance preprocessing modules, the PS end has parallel displacement calculation modules and distance calculation modules, the displacement preprocessing modules are bidirectionally connected with the displacement calculation modules, the displacement preprocessing modules firstly perform downsampling on the collected displacement interference signal, then mix the downsampled displacement interference signal with a carrier base frequency signal and transmit the obtained data to the displacement calculation modules, the displacement calculation modules calculate base frequency phase compensation amounts according to the received data and feed back to the displacement preprocessing modules, the displacement preprocessing modules make the carrier base frequency signal phase synchronous according to the base frequency phase compensation amounts, then mix the downsampled displacement interference signal with a carrier double frequency signal and transmit the obtained data to the displacement calculation modules, the displacement calculation modules calculate double frequency phase compensation amounts according to the received data and feed back to the displacement preprocessing modules, the displacement preprocessing modules make the carrier double frequency signal phase synchronous according to the double frequency phase compensation amounts, then mix the downsampled displacement interference signal with the synchronous carrier base frequency signal and the synchronous carrier double frequency signal respectively and transmit the obtained data to the displacement calculation modules, the displacement calculation modules obtain relative displacement values according to the received data, the distance calculation modules are connected with the distance preprocessing modules, the distance preprocessing modules perform downsampling on the collected distance main interference signal and distance auxiliary interference signal and transmit the obtained data to the distance calculation modules, and the distance calculation modules calculate absolute distance information according to the received data.

7. A demodulation system for simultaneous real-time demodulation of displacement and distance according to claim 6, characterized in that: The distance calculation module comprises a fast peak searching module, an EMD module, a distance phase calculation module, a distance phase unwrapping module, a non-uniform fitting module and a distance result calculation module connected in sequence, the fast peak searching module is used to output the peak position information of the down-sampled distance main interference signal and distance auxiliary interference signal, the EMD module eliminates the direct current amount and envelope fluctuation of the corresponding signal according to the received peak position information, the distance phase calculation module performs arcsin operation on the data processed by the EMD module to extract the signal phase, the distance phase unwrapping module is used to take the inverse of the even interval in the data output by the distance phase calculation module according to the index of the peak contained in the peak position information, to realize the conversion of the triangular wave to the sawtooth wave, to realize the phase unwrapping, to obtain the phase of the distance main interference signal and the phase of the distance auxiliary interference signal, the non-uniform fitting module performs phase domain non-uniform fitting on the phase of the distance main interference signal relative to the phase of the distance auxiliary interference signal by using the least square method, and the distance result calculation module obtains the absolute distance information according to the fitting result, the optical path of the distance auxiliary interferometer, the refractive index of the measurement space and the fiber refractive index.

8. The demodulation system of claim 1 or 2 or 3 or 4, characterized in that: The acquisition unit and the processing unit are implemented based on SoC, the chip used by the SoC is Zynq UltraScale+MPSoC, the displacement calculation module is implemented by a displacement core of the chip, the distance calculation module is implemented by a distance core of the chip, and the displacement core and the distance core are two ARM Cortex-A53 processor cores in the chip Zynq UltraScale+MPSoC, which process data in parallel.

9. The demodulation method of the demodulation system for simultaneously demodulating displacement and distance in real time according to any one of claims 1-8, characterized in that: The method comprises the following steps: In step S1, the acquisition unit acquires the displacement interference signal, the distance main interference signal and the distance auxiliary interference signal respectively, and transmits the acquired signals to the processing unit. In step S2, the processing unit processes the displacement data and the distance data in parallel: the displacement preprocessing module first down-samples the collected displacement interference signals, then mixes the down-sampled displacement interference signals with the carrier base frequency signals and transmits the obtained data to the displacement calculation module, the displacement calculation module calculates the base frequency phase compensation amount according to the received data and feeds back to the displacement preprocessing module, the displacement preprocessing module synchronizes the phase of the carrier base frequency signals according to the base frequency phase compensation amount, then mixes the down-sampled displacement interference signals with the carrier double frequency signals and transmits the obtained data to the displacement calculation module, the displacement calculation module calculates the double frequency phase compensation amount according to the received data and feeds back to the displacement preprocessing module, the displacement preprocessing module synchronizes the phase of the carrier double frequency signals according to the double frequency phase compensation amount, then mixes the down-sampled displacement interference signals with the synchronized carrier base frequency signals and carrier double frequency signals respectively and transmits the obtained data to the displacement calculation module, and the displacement calculation module obtains the relative displacement value according to the received data; the distance calculation module is connected with the distance preprocessing module, the distance preprocessing module down-samples the collected distance main interference signals and distance auxiliary interference signals and transmits the obtained data to the distance calculation module, and the distance calculation module calculates the absolute distance information according to the received data.

10. The demodulation method of claim 9, wherein: The acquisition unit and the processing unit are implemented based on SoC, the chip used by the SoC is Zynq UltraScale+MPSoC ZU7EV, the displacement calculation module is implemented by the displacement core of the chip, the distance calculation module is implemented by the distance core of the chip, and the displacement core and the distance core are two ARM Cortex-A53 processor cores in the chip Zynq UltraScale+MPSoC, which process data in parallel.

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