A spot centroid positioning system and method based on a two-phase digital phase-locked amplifier

Abstract

The application discloses a spot centroid positioning system and method based on a two-phase digital phase-locked amplifier, and belongs to the technical field of photoelectric tracking. The system comprises a four-quadrant photoelectric detector, a signal amplification and collection module, an FPGA signal processing module, a serial communication module and an upper computer module. The four-quadrant photoelectric detector receives a laser signal and converts and outputs four photoelectric current signals. The signal amplification and collection module processes the four photoelectric current signals. The FPGA signal processing module comprises an adaptive frequency tracking module and a two-phase digital phase-locked amplifier module. The serial communication module serially transmits four I / Q effective direct current components to the upper computer module. The upper computer module receives demodulation data and calculates and displays the spot centroid position. The application effectively improves the weak signal extraction and noise resistance, and is mainly used for high-precision spot positioning in an ATP system of space optical communication.

Description

Technical Field

[0001] This invention relates to the field of photoelectric tracking technology, and in particular to a spot centroid positioning system and method based on a dual-phase digital lock-in amplifier. Background Technology

[0002] With the continuous development of space optical communication, laser measurement and control, and precision photoelectric tracking technology, ATP (Acquisition, Tracking and Pointing) system has become the core unit for realizing long-distance laser communication. As the core feedback link of the ATP system, the positioning accuracy, real-time performance and anti-interference capability of the spot position detection directly determine the tracking stability and beam alignment accuracy of the servo system.

[0003] In practical applications, laser detection signals are highly susceptible to interference from sunlight, rain, snow, fog, frost, and stray light. Background noise often completely drowns out the useful, weak laser signal. Simultaneously, the high-speed movement of the carrier carrying the detection equipment generates Doppler frequency shift, causing frequency drift in the laser carrier signal. Conventional spot detection schemes struggle to simultaneously address both strong noise and frequency shift issues. One type of terahertz spot localization system employs a four-quadrant photoelectric detection architecture with discrete hardware, resulting in low system integration, significant signal transmission delays, and a simple internal localization algorithm that cannot adapt to carrier frequency drift scenarios. Furthermore, the system's integrating capacitors are prone to saturation failure after prolonged operation. Another type of laser spot localization scheme based on four-quadrant sensors relies on Kalman filtering for noise reduction and gradient correction algorithms. Algorithm thresholds are set manually based on experience, leading to cumbersome calibration processes, low tolerance for non-uniform light intensity scenarios, and insufficient anti-interference performance in complex stray light environments.

[0004] Therefore, none of the above technologies can simultaneously meet the requirements of weak laser signal extraction, carrier frequency adaptive tracking, highly integrated hardware architecture, and stable positioning in complex lighting environments, making it difficult to meet the high-precision spot centroid detection requirements of the space optical communication ATP system. Summary of the Invention

[0005] The purpose of this invention is to provide a spot centroid positioning system and method based on a dual-phase digital lock-in amplifier, thereby solving the aforementioned technical problems.

[0006] To achieve the above objectives, the present invention provides a spot centroid positioning system based on a dual-phase digital lock-in amplifier, comprising a four-quadrant photodetector, a signal amplification and acquisition module, an FPGA (Field Programmable Gate Array) signal processing module, a serial communication module, and a host computer module; The four-quadrant photodetector receives laser signals and converts them into four photocurrent signals; The signal amplification and acquisition module amplifies, filters and preprocesses the four photocurrent signals and outputs voltage signals, while simultaneously performing analog-to-digital conversion of the four voltage signals in parallel. The FPGA signal processing module includes an adaptive frequency tracking module and a dual-phase digital lock-in amplifier module. The adaptive frequency tracking module automatically identifies the signal carrier frequency and generates a quadrature reference signal. The dual-phase digital lock-in amplifier module combines the quadrature reference signal with phase-sensitive detection and demodulation to obtain four effective DC components of I / Q (In-phase / Quadrature). The serial communication module serially transmits the four effective DC components of I / Q signals to the host computer module. The host computer module receives the demodulated data and calculates and displays the position of the light spot centroid.

[0007] Preferably, the signal amplification and acquisition module includes a transimpedance amplifier circuit, an adjustable gain amplifier circuit, a 50Hz notch filter circuit, an anti-aliasing filter circuit, and a four-channel 16-bit ADC (Analog-to-Digital Converter) connected in sequence. The transimpedance amplifier circuit converts photocurrent into voltage and amplifies it in one stage. The conversion formula is: ; In the formula, V This is the output voltage value. I For input photocurrent, R F It is the feedback resistor and also represents the circuit amplification factor; The adjustable gain amplifier circuit is controlled by the FPGA through the PRCx_A0, PRCx_A1, and PRCx_WR pins to achieve four gain levels of adjustment: 1x, 10x, 100x, and 1000x. The 50Hz notch filter circuit is a Q-adjustable dual-T notch filter circuit composed of dual operational amplifiers, which filters out 50Hz power frequency interference. The Q value represents the quality factor of the notch filter circuit. The anti-aliasing filter circuit is an active low-pass filter composed of dual operational amplifiers, which suppresses high-frequency noise and prevents ADC sampling aliasing. The four-channel 16-bit ADC uses single-ended input. The conversion control, clock, and serial data interface are all connected to the FPGA. The FPGA uniformly completes the sampling control and data reading. The 16 bits represent the ADC's analog-to-digital conversion accuracy.

[0008] Preferably, the adaptive frequency tracking module includes an FFT (Fast Fourier Transform) carrier frequency prediction unit, a DDS (Direct Digital Synthesizer) oscillator, and a squared phase-locked loop; The FFT carrier frequency prediction unit performs an FFT transformation on the digital signal output by the ADC. FFT stands for Fast Fourier Transform, and the maximum value in the frequency domain is extracted to complete a coarse estimation of the carrier frequency. The DDS oscillator generates two quadrature sinusoidal reference signals based on the FFT frequency prediction. DDS stands for Direct Digital Frequency Synthesizer. The square phase-locked loop achieves precise frequency locking based on a rough estimate of the carrier frequency, and corrects the frequency of the DDS output signal until it is in sync with the carrier frequency of the signal under test.

[0009] Preferably, the conversion formula for the frequency control word output by the FFT carrier frequency prediction unit to the DDS oscillator is as follows: ; In the formula, f o The target output frequency of the DDS oscillator output signal. f clk Here, num represents the operating frequency of the FFT, and num represents the position of the maximum spectral line in the FFT. L The number of FFT transformation points. N This refers to the bit width of the binary accumulator in the DDS. K This is the frequency control word corresponding to the center frequency of the DDS.

[0010] Preferably, the dual-phase digital lock-in amplifier module includes two symmetrical phase-sensitive detector units and multi-stage narrowband low-pass filters; each phase-sensitive detector unit includes a multiplier, and the multi-stage narrowband low-pass filters are sequentially a CIC filter (Cascaded Integrator-Comb filter), an ISOP compensation filter, a half-band filter, and an FIR smoothing filter (Finite Impulse Response filter). The CIC filter performs a first-stage downsampling of the signal; CIC stands for cascaded integrator-comb filter. ISOP compensation filter compensates for passband attenuation in CIC filter; A three-and-a-half-stage filter performs two-stage downsampling of the signal; The FIR filter smooths and filters the downsampled signal, outputting pure I / Q DC components. FIR stands for Finite Impulse Response Filter, where I is the in-phase DC component and Q is the quadrature DC component.

[0011] Preferably, the host computer module executes the following solution logic: First, restore the four I / Q DC components received from the serial port, and calculate the single-quadrant spot voltage value. The formula is as follows: ; In the formula, R This is the spot voltage value.A The target signal amplitude, I This refers to the DC component demodulated from channel I. Q The DC component demodulated by the Q-channel. A R The amplitude of the quadrature reference signal; Then based on the four-quadrant spot voltage values R I , R II , R III , R IV Normalization solution for the centroid coordinates of the light spot: ; In the formula, x r To determine the X-axis coordinates of the center of mass of the light spot, y r To determine the Y-axis coordinates of the light spot centroid, R I , R II , R III , R IV The values ​​are the spot voltages calculated in the four quadrants, in order.

[0012] A method for locating the centroid of a light spot based on a dual-phase digital lock-in amplifier, applied to the system described in this invention, includes the following steps: S1, a four-quadrant photodetector receives laser signals and converts them into four photocurrent signals; S2. Perform transimpedance amplification, programmable two-stage gain amplification, 50Hz power frequency notch filtering, and anti-aliasing active low-pass filtering on the four photocurrent signals in sequence, and output four pre-processed voltage signals. S3. A four-channel 16-bit ADC is used to synchronously and parallelly acquire four voltage signals, complete analog-to-digital conversion, and send the digital signals to the FPGA. S4, the FPGA adaptive frequency tracking module processes four digital signals, automatically identifies and locks the signal carrier frequency, and generates two orthogonal sinusoidal reference signals with the same frequency. S5, the dual-phase digital lock-in amplifier performs phase-sensitive detection multiplication operations on the four digital signals and the quadrature reference signal respectively, and filters out the second harmonic component through multi-stage narrowband low-pass filtering to obtain the four effective DC components of I / Q. S6 and FPGA transmit four I / Q DC components to the host computer via serial port. The host computer completes the single-quadrant spot voltage calculation, normalized spot centroid coordinate solution and position visualization display.

[0013] Preferably, the adaptive frequency tracking of S4 specifically includes: S41, the FFT carrier frequency prediction module performs FFT transformation on the digital signal and takes the position of the maximum spectral line in the frequency domain to complete the coarse estimation of the carrier frequency. S42. Convert the rough estimated carrier frequency into the frequency control word of the DDS oscillator, and the DDS initially outputs two quadrature sinusoidal reference signals. S43. The squared phase-locked loop completes precise carrier locking: the two orthogonal sine waves of the DDS are multiplied to generate a double frequency signal, which is then phase-detected with the squared input signal to be measured. The phase difference is adjusted by the loop filter to adjust the DDS frequency control word. After convergence, the DDS outputs an orthogonal reference signal with the same frequency as the carrier to be measured.

[0014] Preferably, the phase-sensitive detection and filtering of S5 specifically includes: S51. The signal under test is multiplied and mixed with two orthogonal reference signals respectively, and two mixed signals are output. The mixed signals contain an effective DC component, a second harmonic component, and a noise product term, wherein the input signal... S ( t )for: ; Orthogonal reference signal S R0 ( t )and S R1 ( t )for: ; In the formula, The angular frequency of the signal to be measured. The initial phase of the signal to be measured. t For the current moment, For environmental noise, The initial phase of the reference signal; S52 sequentially processes the two mixed signals through CIC downsampling, ISOP passband compensation, three-and-a-half-stage filter for secondary downsampling, and FIR smoothing filter to remove the second harmonic component and noise, and outputs a pure I / Q DC component.

[0015] Preferably, the S6 host computer's calculation specifically includes: S61. Parse the serial data from the serial port, restore the I and Q DC components corresponding to the four signals, and calculate the voltage amplitude of the spot in the four quadrants using the spot voltage value calculation formula. S62. Substitute the four-quadrant voltage values ​​and solve for the X and Y axis centroid coordinates of the light spot using the average centroid normalization formula. S63. Real-time display of the calculated centroid coordinates of the light spot and output of the miss position data, which is the offset of the centroid of the light spot relative to the center of the detector.

[0016] Therefore, the beneficial effects of the above-mentioned spot centroid positioning system and method based on a dual-phase digital lock-in amplifier are as follows: 1. Integrating multiple core technologies such as four-quadrant photoelectric detection, adaptive frequency tracking, dual-phase digital phase-locked demodulation, and multi-level signal filtering, it solves the industry pain points of difficulty in extracting weak laser signals under strong background stray light and power frequency noise environments, as well as carrier frequency drift and demodulation distortion caused by carrier Doppler effect, and is suitable for the complex working conditions of space optical communication ATP system.

[0017] 2. The signal link adopts an analog preprocessing noise reduction combined with a digital multi-level narrowband filter collaborative architecture. It relies on a square phase-locked loop to achieve precise carrier frequency locking and combines phase-sensitive detection multiplication to separate the effective components of the target. It can stably extract pure I / Q DC signals in a strong noise substrate, output high signal-to-noise ratio spot detection data, significantly improve the accuracy of spot centroid coordinate calculation, and comprehensively enhance the overall anti-interference performance of the system.

[0018] 3. The entire signal acquisition, frequency tracking, and phase-locked demodulation operation are integrated into a single FPGA for parallel implementation. The hardware integration is high, the signal transmission delay is small, and it has good adaptability to light spots with edge distortion and uneven light intensity distribution. This greatly improves the spot positioning accuracy and long-term operational stability of the system in complex photoelectric detection and beam tracking scenarios.

[0019] 4. The scheme for phase-locked demodulation and positioning of weak spot signals has been improved, providing reliable technical support for long-distance laser measurement and control and precision photoelectric tracking equipment. It can effectively promote the implementation and popularization of dual-phase digital phase-locked demodulation technology in the fields of space optical communication and industrial beam automatic alignment.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating an implementation of a spot centroid positioning system based on a dual-phase digital lock-in amplifier according to the present invention. Figure 2 This is a flowchart illustrating the amplification and filtering process of the system of the present invention. Figure 3 This is a flowchart of the adaptive frequency tracking processing of the system of the present invention; Figure 4 This is a flowchart illustrating the operation of the dual-phase digital lock-in amplifier in the system of this invention. Figure 5 This is a schematic diagram of the overall structure of the system of the present invention; Figure 6 This is an overall flowchart of the method of the present invention.

[0022] Figure Labels 1. Laser; 2. Coupled lens; 3. Four-quadrant photodetector; 4. Amplification and filtering module; 5. ADC acquisition module; 6. Adaptive frequency tracking module; 7. Two-phase digital lock-in amplifier module; 8. Serial communication module; 9. Host computer. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages disclosed in the embodiments of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the embodiments of the present invention and are not intended to limit the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of this application. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout.

[0024] It should be noted that the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion, such as a process, method, system, product, or server that includes a series of steps or units, not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such process, method, product, or device.

[0025] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0026] like Figures 1-6 As shown, the present invention provides a spot centroid positioning system and method based on a dual-phase digital lock-in amplifier. The complete hardware of the spot centroid positioning system based on a dual-phase digital lock-in amplifier used in this embodiment includes: a laser 1, a coupling lens 2, a four-quadrant photodetector 3, a signal amplification and acquisition module 4, an ADC acquisition module 5, an FPGA signal processing module, a serial communication module 8, and a host computer 9. The FPGA main control chip is a Xilinx XC7A200T, and the internal logic of the FPGA is divided into an adaptive frequency tracking module 6 and a dual-phase digital lock-in amplifier module 7.

[0027] After the system is powered on, the optical path configuration of laser 1 and coupling lens 2 is first completed. The position of coupling lens 2 is adjusted to ensure that the modulated laser output from the laser is accurately focused onto the photosensitive surface of the four-quadrant photodetector 3, and the optical path calibration is completed. After the optical path calibration is completed, control commands are sent to the FPGA to pre-configure the gain level of the signal amplification and acquisition module 4, and then the ADC acquisition module 5 is started for synchronous sampling.

[0028] The optical path workflow is as follows: the laser outputs modulated carrier laser → the coupling lens converges and shapes the laser → the four-quadrant photodetector receives the laser spot and outputs four photocurrents → the signal amplification and acquisition module performs analog preprocessing and analog-to-digital conversion → the FPGA performs adaptive frequency tracking and dual-phase lock-in demodulation operations → the serial communication module packages and transmits four I / Q data → the host computer calculates and visualizes the centroid coordinates of the laser spot and outputs the miss distance to the servo system.

[0029] Example 1: Hardware Implementation of Signal Amplification and Acquisition Module The signal amplification and acquisition module 4 consists of a transimpedance amplifier circuit, an adjustable gain amplifier circuit, a 50Hz notch filter circuit, an anti-aliasing filter circuit, and a four-channel 16-bit ADC cascaded together. The transimpedance amplifier circuit is an IV-to-IV conversion circuit built from operational amplifiers, with the conversion formula as follows: ; In the formula, V This is the output voltage value. I For input photocurrent, R F For feedback resistor, at the same time R F As a first-level amplification factor, it converts weak photocurrent into a processable voltage signal.

[0030] The adjustable gain amplifier circuit adopts a programmable gain amplifier. The FPGA can configure the gain level through three independent control pins: PRCx_A0, PRCx_A1, and PRCx_WR. It can switch between four levels of two-stage amplification: 1x, 10x, 100x, and 1000x, to adapt to different detection conditions such as strong light and weak light.

[0031] 50Hz Notch Filter Circuit: Two operational amplifiers are used to build a Q-adjustable dual-T notch filter circuit, where Q represents the circuit quality factor. This circuit is specifically designed to filter out 50Hz power frequency interference in industrial environments and eliminate the modulation interference of power frequency noise on the light spot signal.

[0032] Anti-aliasing filter circuit: An active low-pass filter is built with dual operational amplifiers to suppress high-frequency noise at more than half of the ADC sampling frequency and prevent spectral aliasing distortion during analog-to-digital conversion.

[0033] The 16 bits in the four-channel 16-bit ADC represent the analog-to-digital conversion accuracy. It adopts a single-ended input mode, and the chip's conversion control line, global synchronization clock, and serial data interface are all directly connected to the FPGA. The FPGA outputs a unified synchronization timing sequence to achieve synchronous parallel acquisition and reading of four digital signals from four channels, ensuring that the timing of the signals in the four quadrants is completely aligned and avoiding positioning errors introduced by multi-channel sampling timing deviations.

[0034] Example 2: Implementation of FPGA Adaptive Frequency Tracking Module The FPGA's internal adaptive frequency tracking module includes an FFT carrier frequency prediction unit, a DDS oscillator, and a squared phase-locked loop, where FFT stands for Fast Fourier Transform and DDS stands for Direct Digital Frequency Synthesizer.

[0035] The FFT carrier frequency prediction unit independently performs FFT transformation on the four digital signals output by the ADC, traversing all frequency domain spectrum lines, and extracts the position num of the spectrum line with the maximum amplitude as a rough estimate of the carrier frequency. The conversion formula for the frequency control word output by the FFT carrier frequency prediction unit to the DDS oscillator is as follows: ; In the formula, f o The target output frequency of the DDS oscillator output signal. f clk Here, num represents the operating frequency of the FFT, and num represents the position of the maximum spectral line in the FFT. L The number of FFT transformation points. N This refers to the bit width of the binary accumulator in the DDS. K This is the frequency control word corresponding to the center frequency of the DDS.

[0036] The DDS oscillator receives the frequency control word and initially outputs two orthogonal sinusoidal reference signals. S R0 ( t )and S R1 ( t ),in S R0 ( t (subscript) R 0 represents the reference signal R In the same phase channel of route I, S R1 ( t (subscript) R 1 represents the reference signal R On the main thoroughfare of Q Road.

[0037] The precise frequency locking of the square phase-locked loop is as follows: the two orthogonal sinusoidal signals output by the DDS are multiplied to generate a frequency-doubled signal, and the digital signal under test input by the FPGA is squared simultaneously; the frequency-doubled reference signal and the squared signal under test are sent to the phase detector to obtain the phase difference signal, and the phase difference signal is input to the loop filter to complete the smoothing filtering; the frequency control signal output by the loop filter adjusts the frequency of the DDS oscillator in real time. After multiple iterations and loop convergence, the two orthogonal reference signals output by the DDS are completely in sync with the laser carrier signal, eliminating the frequency deviation caused by the carrier Doppler frequency shift.

[0038] Example 3: Implementation of a Two-Phase Digital Lock-in Amplifier Module The dual-phase digital lock-in amplifier includes two fully symmetrical phase-sensitive detector units and multi-stage narrowband low-pass filters. Each link sequentially connects a CIC filter, an ISOP compensation filter, a half-band filter, and an FIR smoothing filter. The CIC is a cascaded integrator-comb filter, and the FIR is a finite-length unit impulse response filter. I represents the in-phase DC component, and Q represents the quadrature DC component. The phase-sensitive detector multiplication operation specifically includes: The complete expression of the signal to be measured: ; Orthogonal reference signal S R0 ( t )and S R1 ( t )for: ; In the formula, The angular frequency of the signal to be measured. The initial phase of the signal to be measured. t For the current moment, For environmental noise, The initial phase of the reference signal; The signal under test is mixed with two reference signals by a multiplier. The complete mixing expansion is as follows: ; In the formula, S pd0 The I-channel multiplier outputs a mixing signal. S pd1 The Q-channel multiplier outputs a mixing signal. A R For the amplitude of the quadrature reference signal, the first term of the polynomial is the effective DC component, the second term is the second harmonic AC component, and the third term is the product of noise and the reference signal. Due to environmental noise... Since the frequency is unrelated to the carrier frequency, the integral of this product term has a mean of 0, and it can be completely filtered out.

[0039] 2. Multi-stage filtering link processing: S521 Level 1 Downsampling: The CIC filter performs frequency extraction on the mixing signal, reducing the subsequent data processing speed; S522 passband compensation: The ISOP compensation filter compensates for the inherent passband attenuation distortion of the CIC filter; S523 Level 2 Downsampling: A three-and-a-half-stage cascaded filter further compresses the signal bandwidth and reduces the sampling rate; S524 Smoothing Output: FIR smoothing filter narrowband filtering, completely filtering out the second harmonic AC component, and finally outputting stable and pure I and Q DC components.

[0040] Example 4: Implementation of the host computer calculation process The host computer receives the four-channel I and Q DC component serial data packaged by the FPGA via the serial communication module, and completes the calculation of the spot amplitude and the solution of the centroid coordinates in steps, specifically including: For single-quadrant spot voltage, use the formula Calculate the voltage value of the light spot in each quadrant. R In the formula, A The target signal amplitude, I This refers to the DC component demodulated from channel I. Q The DC component demodulated from the Q-path; For the two-dimensional centroid normalization calculation, firstly, assume the spot voltages in the four quadrants are as follows: R I , R II , R III , R IV Then, the centroid coordinates of the light spot are solved using the normalized average centroid equation system, and the formula is: ; In the formula, x r To determine the X-axis coordinates of the center of mass of the light spot, y r The Y-axis coordinates of the light spot centroid are determined, and the ratio of the light intensity difference in the four quadrants to the total light intensity is normalized to eliminate the interference of laser emission power fluctuations on the positioning results.

[0041] The centroid is refreshed in real time via the host computer. x r, y r The coordinate values ​​are used to calculate the off-target position data of the spot centroid relative to the geometric center of the four-quadrant photodetector; the off-target amount is output to the space optical communication ATP servo system in real time as feedback input for beam alignment closed-loop control, and at the same time supports local storage of coordinate data and real-time visual display of spot offset status.

[0042] Example 5: Complete Process of Spot Centroid Localization Method S1. The four-quadrant photodetector receives the laser-modulated laser signal from the laser and converts the light intensity signal to output four independent photocurrent signals. S2. Perform analog preprocessing on the four photocurrents in sequence. The preprocessing order is: first-stage transimpedance amplification → second-stage adjustable gain amplification → 50Hz dual-T notch filter to remove power frequency → anti-aliasing active low-pass filter, and output four stable preprocessed voltage signals. S3. A four-channel 16-bit ADC is used to synchronously and parallelly acquire four voltage signals, synchronously complete analog-to-digital conversion, and send the four digital signals into the FPGA in parallel. S4, the FPGA adaptive frequency tracking module automatically identifies and locks onto the carrier frequency, specifically including: S41, the FFT carrier frequency prediction module performs a fast Fourier transform on the digital signal and takes the maximum spectral line in the frequency domain to complete a coarse estimation of the carrier frequency. S42. Substitute the rough estimated carrier frequency into the complete formula to convert it into the frequency control word of the DDS oscillator. The DDS initially outputs two quadrature sinusoidal reference signals. S43. The squared phase-locked loop completes the precise carrier locking: the two orthogonal sine waves of the DDS are multiplied to generate a double frequency signal, which is then phase-detected with the squared input signal to be measured. The phase difference is adjusted by the loop filter to adjust the DDS frequency control word. After the loop converges, the DDS outputs an orthogonal reference signal with the same frequency as the carrier to be measured. S5, a dual-phase digital lock-in amplifier, completes the demodulation of weak signals, specifically including: S51. The digital signal under test is multiplied and mixed with two orthogonal reference signals to generate a mixed signal containing a product of effective DC, second harmonic AC, and noise. S52. The mixing signal is sequentially passed through CIC downsampling, ISOP passband compensation, three-and-a-half-stage filter for secondary downsampling, and FIR smoothing filter to filter out the second harmonic component and irrelevant noise, and output four channels of pure I / Q effective DC components. S6 and FPGA transmit four-channel I / Q data in a serial package to the host computer via the serial port, specifically including: S61. The host computer parses the serial data from the serial port, restores the I and Q components of each quadrant, and calculates the voltage amplitude R of the single-quadrant spot. S62. Substitute the four-quadrant voltage values ​​and solve for the X and Y axis centroid coordinates of the light spot using the normalized average centroid formula. S63. The host computer interface displays the calculated centroid coordinates of the light spot in real time and outputs the off-target position data for the back-end ATP servo system to track and calibrate the beam in real time.

[0043] This invention's entire system is specifically designed for space optical communication ATP capture, tracking, and alignment systems, adaptable to long-distance laser measurement and control, and precision photoelectric tracking scenarios. It can operate stably under combined conditions of sunlight, stray light intensity and background noise from rain, snow, fog, Doppler frequency drift due to carrier motion, and 50Hz industrial power frequency interference. Compared to terahertz spot positioning systems with discrete hardware architectures, this invention integrates signal acquisition, frequency tracking, and phase-locked demodulation into a single FPGA chip for parallel computation, significantly improving hardware integration and eliminating transmission delays from discrete modules. It eliminates the need for integrator capacitors, thus avoiding capacitor saturation failure issues. Adaptive frequency locking automatically compensates for carrier frequency drift, making it suitable for high-speed moving carriers. Compared to gradient-corrected four-quadrant positioning systems, this invention relies on dual-phase phase-locked locking to extract effective signals of the same frequency, eliminating the need for manual gradient threshold setting and simplifying on-site equipment calibration. Multi-level analog + digital joint filtering significantly improves anti-interference capabilities in complex stray light environments.

[0044] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A spot centroid positioning system based on a two-phase digital lock-in amplifier, characterized in that: It includes a four-quadrant photodetector, a signal amplification and acquisition module, an FPGA signal processing module, a serial communication module, and a host computer module; The four-quadrant photodetector receives laser signals and converts them into four photocurrent signals; The signal amplification and acquisition module amplifies, filters and preprocesses the four photocurrent signals and outputs voltage signals, while simultaneously performing analog-to-digital conversion of the four voltage signals in parallel. The FPGA signal processing module includes an adaptive frequency tracking module and a dual-phase digital lock-in amplifier module. The adaptive frequency tracking module automatically identifies the signal carrier frequency and generates a quadrature reference signal. The dual-phase digital lock-in amplifier module combines the quadrature reference signal with phase-sensitive detection and demodulation to obtain four effective DC components of I / Q. The serial communication module serially transmits the four effective DC components of I / Q signals to the host computer module. The host computer module receives the demodulated data and calculates and displays the position of the light spot centroid.

2. The spot centroid positioning system based on a two-phase digital lock-in amplifier according to claim 1, characterized in that: The signal amplification and acquisition module includes a transimpedance amplifier circuit, an adjustable gain amplifier circuit, a 50Hz notch filter circuit, an anti-aliasing filter circuit, and a four-channel 16-bit ADC, all cascaded together. The transimpedance amplifier circuit converts photocurrent into voltage and amplifies it in one stage. The conversion formula is: ； In the formula, V This is the output voltage value. I For input photocurrent, R F It is the feedback resistor and also represents the circuit amplification factor; The adjustable gain amplifier circuit is controlled by the FPGA through the PRCx_A0, PRCx_A1, and PRCx_WR pins to achieve four gain levels of adjustment: 1x, 10x, 100x, and 1000x. The 50Hz notch filter circuit is a Q-adjustable dual-T notch filter circuit composed of dual operational amplifiers, which filters out 50Hz power frequency interference. The Q value represents the quality factor of the notch filter circuit. The anti-aliasing filter circuit is an active low-pass filter composed of dual operational amplifiers, which suppresses high-frequency noise and prevents ADC sampling aliasing. The four-channel 16-bit ADC uses single-ended input. The conversion control, clock, and serial data interface are all connected to the FPGA. The FPGA uniformly completes the sampling control and data reading. The 16 bits represent the ADC's analog-to-digital conversion accuracy.

3. The spot centroid positioning system based on a two-phase digital lock-in amplifier according to claim 1, characterized in that: The adaptive frequency tracking module includes an FFT carrier frequency prediction unit, a DDS oscillator, and a squared phase-locked loop; The FFT carrier frequency prediction unit performs an FFT transformation on the digital signal output by the ADC. FFT stands for Fast Fourier Transform, and the maximum value in the frequency domain is extracted to complete a coarse estimation of the carrier frequency. The DDS oscillator generates two quadrature sinusoidal reference signals based on the FFT frequency prediction. DDS stands for Direct Digital Frequency Synthesizer. The square phase-locked loop achieves precise frequency locking based on a rough estimate of the carrier frequency, and corrects the frequency of the DDS output signal until it is in sync with the carrier frequency of the signal under test.

4. A spot centroid positioning system based on a dual-phase digital lock-in amplifier according to claim 3, characterized in that: The conversion formula for the frequency control word output by the FFT carrier frequency prediction unit to the DDS oscillator is: ； In the formula, f o The target output frequency of the DDS oscillator output signal. f clk Here, num represents the operating frequency of the FFT, and num represents the position of the maximum spectral line in the FFT. L The number of FFT transformation points. N This refers to the bit width of the binary accumulator in the DDS. K This is the frequency control word corresponding to the center frequency of the DDS.

5. A spot centroid positioning system based on a two-phase digital lock-in amplifier according to claim 1, characterized in that: The dual-phase digital lock-in amplifier module includes two symmetrical phase-sensitive detector units and multi-stage narrowband low-pass filters; each phase-sensitive detector unit contains a multiplier, and the multi-stage narrowband low-pass filters are cascaded with CIC filters, ISOP compensation filters, half-band filters, and FIR smoothing filters in sequence. The CIC filter performs a first-stage downsampling of the signal; CIC stands for cascaded integrator-comb filter. ISOP compensation filter compensates for passband attenuation in CIC filter; A three-and-a-half-stage filter performs two-stage downsampling of the signal; The FIR filter smooths and filters the downsampled signal, outputting pure I / Q DC components. FIR stands for Finite Impulse Response Filter, where I is the in-phase DC component and Q is the quadrature DC component.

6. A spot centroid positioning system based on a dual-phase digital lock-in amplifier according to claim 1, characterized in that: The host computer module executes the following solution logic: First, restore the four I / Q DC components received from the serial port, and calculate the single-quadrant spot voltage value. The formula is as follows: ； In the formula, R This is the spot voltage value. A The target signal amplitude, I This refers to the DC component demodulated from channel I. Q The DC component demodulated by the Q-channel. A R The amplitude of the quadrature reference signal; Then based on the four-quadrant spot voltage values R I , R II , R III , R IV Normalization solution for the centroid coordinates of the light spot: ; In the formula, x r To determine the X-axis coordinates of the center of mass of the light spot, y r To determine the Y-axis coordinates of the light spot centroid, R I , R II , R III , R IV The values ​​are the spot voltages calculated in the four quadrants, in order.

7. A method for locating the centroid of a light spot based on a two-phase digital lock-in amplifier, characterized in that, The system applied to any one of claims 1-6 includes the following steps: S1, a four-quadrant photodetector receives laser signals and converts them into four photocurrent signals; S2. Perform transimpedance amplification, programmable two-stage gain amplification, 50Hz power frequency notch filtering, and anti-aliasing active low-pass filtering on the four photocurrent signals in sequence, and output four pre-processed voltage signals. S3. A four-channel 16-bit ADC is used to synchronously and parallelly acquire four voltage signals, complete analog-to-digital conversion, and send the digital signals to the FPGA. S4, the FPGA adaptive frequency tracking module processes four digital signals, automatically identifies and locks the signal carrier frequency, and generates two orthogonal sinusoidal reference signals with the same frequency. S5, the dual-phase digital lock-in amplifier performs phase-sensitive detection multiplication operations on the four digital signals and the quadrature reference signal respectively, and filters out the second harmonic component through multi-stage narrowband low-pass filtering to obtain the four effective DC components of I / Q. S6 and FPGA transmit four I / Q DC components to the host computer via serial port. The host computer completes the single-quadrant spot voltage calculation, normalized spot centroid coordinate solution and position visualization display.

8. The method for locating the centroid of a light spot based on a two-phase digital lock-in amplifier according to claim 7, characterized in that, S4's adaptive frequency tracking specifically includes: S41, the FFT carrier frequency prediction module performs FFT transformation on the digital signal and takes the position of the maximum spectral line in the frequency domain to complete the coarse estimation of the carrier frequency. S42. Convert the rough estimated carrier frequency into the frequency control word of the DDS oscillator, and the DDS initially outputs two quadrature sinusoidal reference signals. S43. The squared phase-locked loop completes precise carrier locking: the two orthogonal sine waves of the DDS are multiplied to generate a double frequency signal, which is then phase-detected with the squared input signal to be measured. The phase difference is adjusted by the loop filter to control the DDS frequency word. After convergence, the DDS outputs an orthogonal reference signal with the same frequency as the carrier to be measured.

9. A method for locating the centroid of a light spot based on a two-phase digital lock-in amplifier according to claim 7, characterized in that, The phase-sensitive detection and filtering of S5 specifically include: S51. The signal under test is multiplied and mixed with two orthogonal reference signals respectively, and two mixed signals are output. The mixed signals contain an effective DC component, a second harmonic component, and a noise product term, wherein the input signal... S ( t )for: ； Orthogonal reference signal S R0 ( t )and S R1 ( t )for: ； In the formula, The angular frequency of the signal to be measured. The initial phase of the signal to be measured. t For the current moment, For environmental noise, The initial phase of the reference signal; S52 sequentially processes the two mixed signals through CIC downsampling, ISOP passband compensation, three-and-a-half-stage filter for secondary downsampling, and FIR smoothing filter to remove the second harmonic component and noise, and outputs a pure I / Q DC component.

10. A method for locating the centroid of a light spot based on a two-phase digital lock-in amplifier according to claim 7, characterized in that, The S6 host computer's calculations specifically include: S61. Parse the serial data from the serial port, restore the I and Q DC components corresponding to the four signals, and calculate the voltage amplitude of the spot in the four quadrants using the spot voltage value calculation formula. S62. Substitute the four-quadrant voltage values ​​and solve for the X and Y axis centroid coordinates of the light spot using the average centroid normalization formula. S63. Real-time display of the calculated centroid coordinates of the light spot and output of the miss position data, which is the offset of the centroid of the light spot relative to the center of the detector.

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