A modulation intensity distribution control method for multi-frequency fusion I-TOF ranging system

Abstract

The present invention discloses a modulation intensity distribution control method for a multi-frequency fusion I-TOF ranging system, which belongs to the field of optical fiber sensing. The method comprises the following steps: (1) a multi-frequency modulation signal generated by a signal generator is used to intensity modulate a laser output light signal through an intensity modulator; (2) the intensity-modulated light signal is divided into two paths, a reference light and a measurement light, the measurement light is emitted to a target object, and the reference light is directly received by a photodetector and then enters a compensation unit; (3) the compensation unit converts the collected reference path electrical signal data into a digital signal, calculates the compensation reference amount of each modulation frequency signal based on the digital signal and feeds it back to the signal generator to control the intensity distribution of each modulation frequency signal; (4) repeating the above steps to realize real-time control of the modulation intensity distribution of the multi-frequency modulation signal, so that each modulation frequency signal maintains the same signal-to-noise ratio. The present invention can suppress the adverse effects of the difference in signal-to-noise ratio of different frequency modulation signals on the accuracy of multi-frequency fusion ranging.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing, and in particular to a modulation intensity distribution control method for a multi-frequency fusion I-TOF ranging system. Background Art

[0002] The I-TOF ranging system is based on the principle of phase method and has the advantages of high speed, high precision, and large range. It is widely used in important fields such as intelligent construction, rail transportation, and digital industry.

[0003] The multi-frequency fusion I-TOF ranging system uses the superposition of multiple different modulation frequencies to resolve the contradiction between high precision and large unambiguous distance that exists in a single modulation frequency. It expands the measurement range through low-frequency modulation signals and improves measurement accuracy through high-frequency modulation signals, and has important engineering application value.

[0004] However, in the multi-frequency fusion I-TOF ranging system, the inherent characteristics of optoelectronic devices such as modulators, photodetectors (PDs), and transimpedance amplifiers (TIAs) are frequency-dependent, and coupled with the influence of environmental factors, different frequency signals are subject to different and time-varying gains or attenuations in the system, resulting in different signal-to-noise ratios of each modulated frequency signal during the phase difference demodulation process, ultimately affecting the accuracy and stability of multi-frequency fusion. Summary of the Invention

[0005] In order to solve the above problems, the present invention proposes a modulation intensity distribution control method for a multi-frequency fusion I-TOF ranging system. Through signal acquisition, data processing and feedback control, the implementation and control of the distribution of different modulation signal intensities are realized, effectively suppressing the differentiated fluctuations of different modulation signal intensities caused by the characteristics of the optoelectronic devices themselves and environmental factors in the system, thereby simultaneously improving the accuracy and stability of the system's ranging.

[0006] The present invention proposes the following technical solutions:

[0007] A multi-frequency fusion I-TOF ranging system modulation intensity distribution control method includes the following steps:

[0008] (1) The multi-frequency modulation signal generated by the signal generator is used to modulate the intensity of the laser output light signal through the intensity modulator, and the total signal intensity of the signal generator is fixed;

[0009] (2) The intensity-modulated optical signal is divided into two paths, reference light and measurement light, by a beam splitter. The measurement light is emitted to the target object and returns, while the reference light is directly received by a photodetector, converted into an electrical signal by the photodetector, and then enters the compensation unit.

[0010] (3) The compensation unit converts the collected reference circuit electrical signal data into a digital signal, calculates the compensation reference amount of each modulation frequency signal based on the digital signal, and feeds the compensation reference amount back to the signal generator to adjust the intensity distribution of each modulation frequency signal;

[0011] (4) Repeat steps (1) to (3) to achieve real-time control of the modulation intensity distribution of the multi-frequency modulation signal so that the signal-to-noise ratio of each modulation frequency signal remains the same.

[0012] Furthermore, the relationship between the total signal strength of the signal generator and the signal strength of each modulation frequency is as follows:

[0013]

[0014] Among them, A i represents the distribution strength of the i-th modulation frequency signal generated by the signal generator, n represents the number of modulation frequency signals in the multi-frequency modulation signal, and A0 represents the total signal strength of the signal generator.

[0015] Furthermore, the compensation unit includes:

[0016] An analog-to-digital converter (ADC) is used to convert the collected reference circuit electrical signal data into a digital signal;

[0017] A field programmable gate array (FPGA) is used to calculate a compensation reference value of each modulation frequency signal based on a digital signal output by an ADC, and feed the compensation reference value back to a signal generator.

[0018] Furthermore, the calculation process of the FPGA in the compensation unit includes:

[0019] A window function is used to periodically intercept multiple segments of digital signals converted from reference circuit electrical signals, and the intercepted multiple segments of signals are superimposed and averaged to suppress the influence of noise, thereby obtaining a superimposed and averaged signal;

[0020] The signal after superposition and averaging is transformed using Fourier transform to obtain the signal spectrum information, and the signal strength S corresponding to each modulation frequency signal is extracted respectively. i , and at the same time obtain the spectrum noise floor N0 of the superimposed and averaged signal, and calculate the signal-to-noise ratio SNR of each modulation frequency signal i =S i / N0;

[0021] The difference between the signal-to-noise ratio of each modulation frequency signal and the average signal-to-noise ratio is calculated as a compensation reference amount for each modulation frequency signal.

[0022] Furthermore, the length L of the window function satisfies the following conditions at the same time:

[0023] L=2 n , n is a positive integer;

[0024] L covers at least one full cycle of the lowest modulation frequency;

[0025] All modulation frequencies are divisible by the frequency resolution of the compensation unit, where the frequency resolution = ADC sampling frequency / L.

[0026] Furthermore, the sampling frequency of the ADC is greater than 4 times the maximum modulation frequency.

[0027] Furthermore, the signal generator includes:

[0028] A field programmable gate array (FPGA) is used to generate a digital signal with intensity distribution information according to a compensation reference amount of each modulation frequency signal;

[0029] A direct digital frequency synthesizer DDS is used to obtain the digital signal with intensity distribution information and generate an analog multi-frequency modulation signal.

[0030] Furthermore, the multi-frequency fusion I-TOF ranging system achieves ranging by demodulating the phase difference between the reference light and the measurement light returned from the target object.

[0031] The beneficial effects of the present invention are as follows: by real-time acquisition of the signal-to-noise ratio of multi-frequency modulation signals and real-time regulation of the modulation signal intensity distribution of the multi-frequency fusion I-TOF ranging system, the present invention effectively reduces the different degrees of gain or attenuation of different frequency signals produced by the optoelectronic devices in the system due to their own characteristics or environmental factors, thereby suppressing the adverse effects of the differences in the signal-to-noise ratios of different frequency modulation signals on the multi-frequency fusion ranging accuracy, and improving the ranging accuracy and stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Schematic diagram of the modulation signal intensity distribution and control principle in the multi-frequency fusion I-TOF ranging system.

[0033] Figure 2 This is the time domain diagram of the signal collected before control.

[0034] Figure 3 This is the time domain diagram of the signal in a single window before adjustment.

[0035] Figure 4 This is the signal time domain diagram after superposition and averaging of multiple window signals before control.

[0036] Figure 5 It is the spectrum diagram of the signal before regulation.

[0037] Figure 6 is the spectrum of the signal after regulation. DETAILED DESCRIPTION

[0038] The present invention collects signal data in real time, obtains a compensation reference after data processing, and uses the compensation reference to feedback control the signal generating unit to achieve real-time regulation of the intensity distribution of different frequency modulation signals, thereby simultaneously improving the accuracy and stability of multi-frequency fusion ranging.

[0039] The real-time control method for the modulation signal intensity distribution of the multi-frequency fusion I-TOF ranging system of the present invention is described in detail below with reference to the accompanying drawings.

[0040] like Figure 1 As shown in the multi-frequency fusion I-TOF ranging system, the laser output light signal enters the intensity modulator, the signal generator generates a multi-frequency modulation signal which enters the intensity modulator, and the intensity modulator is used to apply intensity modulation to the light signal according to the multi-frequency modulation signal. That is, the multi-frequency modulation signal generated by the signal generator modulates the intensity of the laser output light signal through the intensity modulator, and the frequency of the modulation signal is f i , i=1,2,…,n, the corresponding signal strength is A i , i=1,2,…,n, the total signal strength that the signal generator can generate is A0, then Where n represents the number of multi-frequency modulation signals.

[0041] The intensity-modulated optical signal is divided into two paths, reference light and measurement light, by a beam splitter. The measurement light is emitted to the target object, and the reference light is directly received by the photodetector. After being converted into an electrical signal by the photodetector, it enters the compensation unit, which collects the electrical signal data of the reference path. The ranging principle of the multi-frequency fusion I-TOF ranging system is to achieve ranging by demodulating the phase difference between the reference light and the measurement light. Multiple modulation frequencies are used to solve the contradiction between the measurement accuracy and the unambiguous measurement range of a single frequency. When the signal-to-noise ratio of different modulation frequencies is the same, the accuracy and stability of the multi-frequency fusion ranging can achieve the best balance.

[0042] In the present invention, a compensation reference is obtained after signal acquisition and data processing in the compensation unit, and is transmitted to the signal generator to complete feedback control, thereby achieving real-time regulation of signal strength distribution at different modulation frequencies.

[0043] In a specific implementation of the present invention, the compensation unit uses a window function to periodically intercept multiple segments of digital signals converted from the reference circuit electrical signal, and the intercepted signals are recorded as Wm, m = 1, 2, ..., k; the length L of the window function satisfies: (1) L = 2 n, n is a positive integer, (2) L covers at least one full cycle of the lowest modulation frequency, (3) all modulation frequencies can be divided by the frequency resolution of the compensation unit (i.e., ADC sampling frequency / L). In this example, L = 4096. Since the noise in the data conforms to the Gaussian distribution and is independent and identically distributed, the noise effect can be effectively suppressed by superimposing and averaging the intercepted signals to obtain more accurate spectrum intensity information. The superimposed and averaged signal is recorded as Existence W mean The width is smaller than any W m , W mean It can represent the common part of k periodic data after eliminating noise interference, which is used as the basis for compensation; then the signal spectrum information is obtained by Fourier transform on the superimposed and averaged signal, and the signal strength S corresponding to fi is extracted respectively. i At the same time, the spectrum noise floor N0 of the superimposed and averaged signal can be obtained, and the SNR i =S i / N0 calculates the signal-to-noise ratio corresponding to each modulation frequency; calculates the average signal-to-noise ratio Further calculate the difference between the signal-to-noise ratio of each modulation frequency and the average signal-to-noise ratio ΔSNR i =SNR i -SNR mean , then ΔSNR i That is, the compensation amount of each modulation frequency; the compensation amount of each modulation frequency is fed back to the signal generator to realize real-time regulation of the signal strength distribution of each modulation frequency, so that each modulation frequency maintains the same signal-to-noise ratio in the final phase difference detection process to obtain the optimal multi-frequency fusion ranging result.

[0044] In this example, the laser wavelength used is 1550nm, the intensity modulator is a lithium niobate modulator, the beam splitter is a 1:99 beam splitter, the energy share of the reference path is 1%, the photodetector is an InGaAs PIN, and the compensation unit consists of an analog-to-digital converter (ADC) and a field-programmable gate array (FPGA). The ADC is used to convert the reference path electrical signal data into a digital signal, and the ADC sampling frequency should be greater than 4 times the maximum modulation frequency. The FPGA is used to process the digital signal to obtain the compensation amount for each modulation frequency.

[0045] The signal generator consists of a direct digital synthesizer (DDS) and a field-programmable gate array (FPGA). The FPGA generates a digital signal with intensity distribution information based on the compensation amount of each modulation frequency, while the DDS takes the digital signal output by the FPGA and generates an analog multi-frequency modulation signal. In this example, the multi-frequency modulation signal is a three-frequency signal with frequencies of 2.5 MHz, 20 MHz, and 500 MHz. By adjusting the intensity distribution of the three-frequency signals in real time, the problem of different gain or attenuation of different frequency signals caused by the optoelectronic devices in the system due to their own characteristics or environmental factors is reduced. This reduces the adverse effect of different signal-to-noise ratios of the different frequency modulation signals on the accuracy of multi-frequency fusion ranging, thereby improving the accuracy and stability of the system's ranging.

[0046] like Figure 2 The figure shows the time domain diagram of the signal collected before the control, which is generated by the ADC in the compensation unit. The horizontal axis represents the number of sampling points, and the vertical axis represents the digital value of the sampled data. Figure 3 Shown is a time domain plot of the signal within a single window before regulation.

[0047] like Figure 4 The figure shows the signal time domain diagram after superimposing and averaging multiple window signals before control. It can be seen that superposition and averaging can effectively suppress noise.

[0048] like Figure 5 The figure shows the spectrum of the signal before modulation, where the horizontal axis represents frequency and the vertical axis represents intensity. It can be seen that there are significant differences in the intensity and signal-to-noise ratio of the three modulation frequencies, which will affect the accuracy and stability of multi-frequency fusion and lead to ranging problems such as "wavelength hopping". Figure 6 The figure shows the spectrum of the signal after regulation. It can be seen that the compensation method of the present invention effectively regulates the distribution of signal strengths at different modulation frequencies, achieving uniform distribution of signal strengths at different frequency components.

[0049] In summary, the intensity distribution of the modulation signal in the multi-frequency fusion I-TOF ranging system can be controlled in real time by the method of the present invention. The control method proposed in the present invention is based on simulation and experiments and has a good compensation effect.

[0050] The above is merely a preferred embodiment of the present invention, and the scope of the rights claimed by the present invention is not limited thereto. The present invention may also have various other embodiments. Those skilled in the art may make corresponding changes and modifications based on the present invention without departing from the spirit and essence of the present invention. Such changes and modifications shall fall within the scope of protection of the appended claims.

Claims

1. A method for controlling the modulation intensity distribution of a multi-frequency fusion I-TOF ranging system, characterized in that: The following steps are involved: (1) The multi-frequency modulation signal generated by the signal generator is used to modulate the intensity of the laser output light signal through the intensity modulator, and the total signal intensity of the signal generator is fixed; (2) The intensity-modulated optical signal is divided into two paths, reference light and measurement light, by a beam splitter. The measurement light is emitted to the target object and returns, while the reference light is directly received by a photodetector, converted into an electrical signal by the photodetector, and then enters the compensation unit. (3) The compensation unit converts the collected reference circuit electrical signal data into a digital signal, calculates the compensation reference amount of each modulation frequency signal based on the digital signal, feeds the compensation reference amount back to the signal generator, and adjusts the intensity distribution of each modulation frequency signal. The compensation unit includes: An analog-to-digital converter (ADC) is used to convert the collected reference circuit electrical signal data into a digital signal; A field programmable gate array (FPGA) is used to calculate a compensation reference value for each modulation frequency signal based on the digital signal output by the ADC, and feed the compensation reference value back to the signal generator; The calculation process of the FPGA in the compensation unit includes: using a window function to periodically intercept multiple segments of digital signals converted from the reference circuit electrical signal, superimposing and averaging the intercepted multiple segments of the signal to suppress noise, and obtaining a superimposed and averaged signal; The signal after superposition and averaging is transformed using Fourier transform to obtain the signal spectrum information, and the signal strength S corresponding to each modulation frequency signal is extracted respectively. i , and at the same time obtain the spectrum noise floor N0 of the superimposed and averaged signal, and calculate the signal-to-noise ratio SNR of each modulation frequency signal i =S i / N0; Calculating the difference between the signal-to-noise ratio of each modulation frequency signal and the average signal-to-noise ratio as a compensation reference for each modulation frequency signal; (4) Repeat steps (1) to (3) to achieve real-time control of the modulation intensity distribution of the multi-frequency modulation signal so that the signal-to-noise ratio of each modulation frequency signal remains the same.

2. The modulation intensity distribution control method of the multi-frequency fusion I-TOF ranging system according to claim 1 is characterized in that: The relationship between the total signal strength of the signal generator and the signal strength of each modulation frequency is as follows: Among them, A i represents the distribution strength of the i-th modulation frequency signal generated by the signal generator, n represents the number of modulation frequency signals in the multi-frequency modulation signal, and A0 represents the total signal strength of the signal generator.

3. The modulation intensity distribution control method of the multi-frequency fusion I-TOF ranging system according to claim 1 is characterized in that: The length L of the window function satisfies the following conditions at the same time: L=2 n , n is a positive integer; L covers at least one full cycle of the lowest modulation frequency; All modulation frequencies are divisible by the frequency resolution of the compensation unit, where the frequency resolution = ADC sampling frequency / L.

4. The modulation intensity distribution control method of the multi-frequency fusion I-TOF ranging system according to claim 1 is characterized in that: The sampling frequency of the ADC is greater than 4 times the maximum modulation frequency.

5. The modulation intensity distribution control method of the multi-frequency fusion I-TOF ranging system according to claim 1 is characterized in that: The signal generator comprises: A field programmable gate array (FPGA) is used to generate a digital signal with intensity distribution information according to a compensation reference amount of each modulation frequency signal; A direct digital frequency synthesizer DDS is used to obtain the digital signal with intensity distribution information and generate an analog multi-frequency modulation signal.

6. The modulation intensity distribution control method of the multi-frequency fusion I-TOF ranging system according to claim 1 is characterized in that: The multi-frequency fusion I-TOF ranging system achieves ranging by demodulating the phase difference between the reference light and the measurement light returned from the target object.

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