Hardware real-time decoupling multi-degree-of-freedom measurement system based on double femtosecond laser

Abstract

The invention discloses a hardware real-time decoupling multi-degree-of-freedom measurement system based on double femtosecond laser, which is characterized in that laser emitted by a measurement light source enters a beam splitter through a circulator and is decomposed into three independent measurement light beams, and the three independent measurement light beams are respectively projected to corresponding pyramid reflector arrays on a cooperative target through a first collimator, a second collimator and a third collimator. After reflected light returns to the beam splitter through an original light path to be combined, laser carrying three paths of distance measurement information and local oscillator laser are subjected to asynchronous optical sampling in the cross-correlation module and converted into broadened laser signals, the broadened laser signals enter the photoelectric conversion module to be converted into analog signals, the analog signals are resolved by the signal processing module, and the broadened laser signals are transmitted to the light source module. Wherein the hardware processing layer adopts an FPGA (Field Programmable Gate Array) to realize real-time noise filtering and pulse interval detection; the software algorithm layer completes peak value fitting, distance value calculation and multi-degree-of-freedom solution through a Gaussian algorithm.

Description

Technical Field

[0001] The present invention relates to the field of laser posture measurement, and in particular to a hardware real-time decoupling multi-degree-of-freedom measurement system based on double femtosecond lasers. Background Art

[0002] As space exploration missions evolve toward high-precision, distributed collaboration, the precise, synchronized determination of spacecraft relative positions has become a core technical bottleneck for deep-space observations and on-orbit operations. In the field of space telescope arrays, interferometric imaging systems composed of multiple optical units must overcome the diffraction limit of a single mirror through beam combining. Their imaging resolution directly depends on the synchronization accuracy of the relative positions between the sub-telescopes. When baseline lengths reach kilometers, to ensure λ / 20 optical path consistency and micro-arc-level pointing stability, the intersatellite relative position and attitude measurement errors must be controlled to nanometer-micro-arc levels. This requirement is also prominent in satellite formation collaborative missions. For example, a distributed synthetic aperture radar satellite constellation relies on submillimeter-level baseline stability to construct an equivalent large-aperture antenna to achieve meter-level surface deformation monitoring. A gravity field measurement satellite formation uses intersatellite laser ranging to invert Earth mass transfer. Its micro-gamma-level gravity anomaly detection accuracy requires a relative position resolution of nanometers.

[0003] In order to meet the stringent requirements for posture synchronization at the above-mentioned work sites, the aerospace industry has developed a series of relative posture measurement technologies for space cooperation targets, mainly including: visual measurement technology, laser measurement technology, and multi-sensor fusion technology. Visual measurement technology has advantages such as high resolution and non-contact, but it has disadvantages such as susceptibility to interference from ambient light, low real-time performance, and low accuracy. Laser measurement technology uses lasers to continuously scan spacecraft by emitting laser pulses or continuous waves to obtain point cloud data of the scanned object to solve spatial coordinates. The posture accuracy of the laser measurement method is much higher than that of visual measurement, and it has better robustness and real-time performance. However, due to the limitations of laser pulse width, this method still faces significant technical challenges in achieving higher ranging resolution and improving real-time detection capabilities.

[0004] The appearance of femtosecond laser provides a new direction for the development of laser measurement. In time domain, it is characterized by femtosecond-level ultra-short pulse, and in frequency domain, it is characterized by equal-interval wide-band spectrum, which constitutes a natural calibration benchmark for multi-dimensional pose parameters. The femtosecond laser pose measurement system not only overcomes the problem of insufficient dynamic tracking rate in traditional laser measurement, but also realizes the three-dimensional positioning with nanometer resolution and the angle calculation with micro-radian level through the synchronous acquisition mechanism of fusing absolute distance and angle parameters, and establishes a traceable dynamic measurement system. The femtosecond laser absolute distance measurement technology based on time-of-flight method has the characteristics of high stability, fast measurement speed and easy integration, which can meet the demand of high-precision pose measurement in the field of aerospace. It is very important to expand from single target length measurement to space multi-target three-dimensional measurement in order to realize high-precision pose measurement of cooperative targets. However, the inherent characteristics of femtosecond laser, such as ultra-short pulse width and ultra-wide spectrum, make it difficult for a single ranging system to realize effective path separation and signal matching in a multi-target scene, which restricts the realization of multi-degree-of-freedom collaborative measurement capability. SUMMARY

[0005] The application provides a kind of based on double femtosecond laser hardware real-time decoupling multi-degree-of-freedom measurement system, the application is realized by introducing different delay optical fiber delay line the time domain discrimination of multiple measurement pulses, with field programmable gate array (FPGA) hardware for signal denoising, with software to realize peak analysis and distance calculation, realize the real-time solution of multiple distance and complete the integration design of system, the application solves the precision of laser pose measurement technology is low, real-time is poor, robustness is poor and the limitation such as complex structure, see the description as follows:

[0006] A kind of based on double femtosecond laser hardware real-time decoupling multi-degree-of-freedom measurement system, the system includes:

[0007] The measurement laser emitted by the measurement light source enters the beam splitter after the circulator, and is divided into three measurement lasers, which enter the first corner cube reflector, the second corner cube reflector and the third corner cube reflector on the cooperative target from the first collimator, the second collimator and the third collimator respectively through the optical fiber.

[0008] The three-way laser is reflected by the cooperative target and returns to the beam splitter along the original path, and the laser carrying three-way measurement information returns to the circulator along the original path to complete the beam combination, and the laser carrying three-way measurement information enters the cross-correlation module to generate the local laser based on the local light source for asynchronous optical sampling.

[0009] The measurement laser is expanded in time domain, the period of the expanded laser signal is obtained, the expanded laser signal is converted into an analog signal by the photoelectric conversion module, and the analog signal is processed by the signal processing module to obtain three ranging values, and each degree of freedom is calculated based on the ranging values.

[0010] The period of the expanded laser signal is:

[0011]

[0012] Where Δf r is the repetition frequency difference between the local oscillator light source and the measurement light source.

[0013] The system calculates the distance information corresponding to the three targets, thereby obtaining multiple degrees of freedom of the targets.

[0014] The signal processing module uses a dual-core CPU, where CPU0 is responsible for reading the pre-processed data to calculate the distance value and multiple degrees of freedom, and CPU1 is responsible for controlling the three serial port IPs in the FPGA to communicate with the laser driver and temperature control modules via serial ports.

[0015] Furthermore, the signal processing module is:

[0016] After asynchronous optical sampling and stretching, the laser pulse is converted into an electrical signal by an avalanche diode. After the reference pulse and the measurement pulse are A / D converted, they are pre-processed by two IP cores respectively. The pre-processing part needs to filter out signal noise based on dynamic thresholds, retain valid sampling points on the stretched pulse for peak fitting, and count the sampling intervals.

[0017] When the sampled signal is greater than the set threshold, the preprocessing IP retains the sampled value; when the sampled signal is less than the threshold, the sent data is set to 0 to filter out all noise.

[0018] The pre-processing IP sends data to the FIFO and writes it to the shared memory via DMA. The data sent by the measurement signal pre-processing IP is as follows:

[0019] After detecting the rising edge of the reference signal valid flag, prepare to send data. If the measurement signal is valid, send the pulse interval clock count, and then send the valid sampling point. Until the measurement signal is no longer valid, send the stop bit to complete the transmission of a complete frame of data. Each time a group of data is sent, add 1 to the sending count flag. When the sending count flag is equal to the measurement target number, continue to wait for the rising edge of the reference signal. Otherwise, continue to wait for the measurement signal to reach the threshold before sending the sampling time interval and sampling point.

[0020] After completing the complete sampling of a pulse, the preprocessing IP transfers the data to the FIFO, which is then moved to the shared memory by DMA. The ARM core software reads the data in the shared memory and performs Gaussian fitting to obtain the peak position and calculate the actual distance.

[0021] The beneficial effects of the technical solution provided by the present invention are:

[0022] 1. This invention utilizes a dual-femtosecond laser asynchronous optical sampling method to improve the accuracy of absolute distance measurement. The measurement speed corresponds to the repetition rate difference of the dual-comb laser, which improves the dynamic tracking rate of posture measurement and establishes a high-real-time measurement system with on-site traceability.

[0023] 2. By introducing an optical fiber delay line to establish time-domain distinguishable features for the measurement pulse, the present invention achieves path separation and signal matching for a single ranging system in a multi-target scenario, improving the system's multi-path ranging collaborative measurement capability and thus increasing the integration of the laser multi-degree-of-freedom measurement system.

[0024] 3. The present invention can be widely used in high-precision parallel measurement of absolute distances of multiple targets in space at industrial production sites, real-time monitoring of the position and posture of aerospace cooperation targets, precise synchronization of relative position and posture of spacecraft, and high-precision control of distributed spacecraft dynamic arrays. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic diagram of the structure of the dual-femtosecond laser integrated multi-degree-of-freedom measurement system based on the time-of-flight method;

[0026] Figure 2 Schematic diagram of the signal processing system-on-chip architecture design;

[0027] Figure 3 Flowchart for femtosecond laser repetition rate calculation.

[0028] In the accompanying drawings, the components represented by the reference numerals are as follows:

[0029] 1: Multi-degree-of-freedom measurement system structure; 2: Internal structure of the integrated rangefinder;

[0030] 11: integrated rangefinder; 12: beam splitter;

[0031] 13: Host computer; 14: First collimator;

[0032] 15: second collimator; 16: third collimator;

[0033] 17: first retroreflector; 18: second retroreflector;

[0034] 19: the third retroreflector;

[0035] 21: Measurement light source; 22: Local oscillator light source;

[0036] 23: circulator; 24: cross-correlation module;

[0037] 25: Photoelectric conversion module; 26: Signal processing module. DETAILED DESCRIPTION

[0038] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention are described in further detail below.

[0039] A hardware real-time decoupling multi-degree-of-freedom measurement system based on dual femtosecond lasers, such as Figure 1 As shown, it includes: an integrated rangefinder 11, a beam splitter 12, a host computer 13, a first collimator 14, a second collimator 15, a third collimator 16, a first corner reflector 17, a second corner reflector 18 and a third corner reflector 19 on the cooperative target.

[0040] Depending on the number of degrees of freedom required to be measured, the number of target cones and collimators on the cooperative target can be changed. Figure 1 The internal structure of the integrated rangefinder includes: a measuring light source 21 , a local oscillator light source 22 , a circulator 23 , a cross-correlation module 24 , a photoelectric conversion module 25 and a signal processing module 26 .

[0041] The working process of the multi-degree-of-freedom measurement system is as follows: the measurement laser emitted by the measurement light source 21 enters the beam splitter 12 after passing through the circulator 23 and is divided into three beams of measurement laser. The three beams of measurement laser are incident on the first corner reflector 17, the second corner reflector 18, and the third corner reflector 19 on the cooperative target through optical fibers from the first collimator 14, the second collimator 15, and the third collimator 16 respectively. After being reflected by the cooperative target, these three laser beams return to the beam splitter 12 along the original path, are combined in the beam splitter 12, and then return to the circulator 23 in the integrated rangefinder 11 along the original path. At this time, the laser beam carrying the three-way measurement information will enter the cross-correlation module 24 and be asynchronously optically sampled by the local oscillator laser generated by the local oscillator light source 22. At this time, the measurement laser beam will be broadened in the time domain, and the period of the broadened laser signal is:

[0042]

[0043] Where Δf r is the repetition frequency difference between the local oscillator light source 22 and the measurement light source 21.

[0044] After the stretched laser signal enters the photoelectric conversion module 25 , it is converted into an analog signal, which is further processed by the signal processing module 26 and finally converted into a three-way ranging value.

[0045] When multiple targets are too close together, causing measurement pulses to overlap in the time domain, a measurement dead zone occurs. Therefore, the upper limit of the number of multi-target measurements in this embodiment of the present invention is determined by the repetition frequency difference and the signal pulse width. Because the repetition frequency difference between the local oscillator light source 22 and the measurement light source 21 is only slightly different, and the femtosecond laser has ultrashort pulse characteristics, this embodiment of the present invention can measure up to hundreds of targets in parallel. By processing the stretched reflected laser signal, the distance information corresponding to the three targets can be calculated, and thus the multiple degrees of freedom of the target can be calculated.

[0046] The detailed on-chip architecture design of the signal processing module 26 is as follows Figure 2 As shown, the signal processing module 26 uses FPGA as the core signal processing component to perform real-time decoupling of the laser pulse signal, and uses a dual-core CPU for system control and algorithm implementation, wherein CPU0 is responsible for reading the pre-processed data to calculate the distance value and multiple degrees of freedom, and CPU1 is responsible for controlling the three serial port IPs in the FPGA to communicate with the laser drive and temperature control modules via serial ports.

[0047] The core board's workflow is as follows: After asynchronous optical sampling and stretching, the laser pulse is converted into an electrical signal by an avalanche diode (APD). After analog-to-digital conversion, the reference and measurement pulses are preprocessed by two separate IP cores. These preprocessing steps filter out signal noise based on a dynamic threshold, retain valid sampling points on the stretched pulse for peak fitting, and clock the sampling interval. When the sampled signal exceeds the set threshold, the preprocessing IP retains the sampled value. When the sampled signal falls below the threshold, the transmitted data is set to 0 to filter out all noise.

[0048] To interconnect data between the FPGA and the ARM processor, the preprocessing IP needs to send data to a FIFO, which is then written to shared memory via DMA. The measurement signal preprocessing IP transmits data as follows: After detecting a rising edge in the reference signal valid flag, it prepares to transmit data. If the measurement signal is valid, it sends a pulse interval clock count. Valid sampling points are then transmitted until the measurement signal is no longer valid (i.e., the sampling signal falls below the threshold), at which point a stop bit is sent, completing the transmission of a complete frame of data. Each time a set of data is transmitted, the transmit count flag is incremented. When the transmit count flag equals the target number of measurements, it waits for a rising edge on the reference signal. Otherwise, it waits for the measurement signal to reach the threshold before transmitting the sampling interval and sampling points.

[0049] After completing the sampling of a pulse, the preprocessing IP transfers the data to the FIFO, which is then moved to shared memory by DMA. When calculating the distance, the ARM core software reads the data from shared memory and performs a Gaussian fit to determine the peak position. The actual distance can then be calculated using the following formula:

[0050]

[0051] Wherein, c is the speed of light in vacuum, n g is the refractive index of air, f r is the repetition frequency of the signal light source, t Refi is the corresponding time of each reference pulse, t Tari is the corresponding time of the target pulse, t' Refi is the corresponding time of the secondary cycle reference pulse.

[0052] According to the three-way ranging values obtained by solving, the following formula is substituted, so that the pitch angle and the yaw angle can be calculated.

[0053]

[0054] Wherein, L1 and L2 are the distances from the vertex of the second corner reflector 18 to the vertex of the first corner reflector 17 and the third corner reflector 19 respectively, and the values of L1 and L2 are calibrated by using the Multiline system; Δl 12 , Δl 23 respectively represent the distance change of the vertex of the first corner reflector 17 and the third corner reflector 19 along the z-axis direction relative to the vertex of the second corner reflector 18 when the target mirror deflects around the x-axis and the y-axis directions respectively.

[0055] As Figure 3 shown, in order to ensure the real-time traceability of the ranging value, the embodiment of the application compiles a repetition frequency counting module IP core based on FPGA, references the repetition frequency of the laser to the standard square wave signal of the rubidium atomic clock, and calculates the repetition frequency of the two lasers according to the pulse number of the two femtosecond pulses and the rubidium clock signal within a certain gate time.

[0056] In summary, the embodiment of the application studies a hardware real-time decoupling multi-degree-of-freedom measurement system based on double femtosecond lasers, realizes multi-target identification by constructing time-domain distinguishable characteristics with different delays, designs a real-time solving algorithm based on FPGA, and realizes the simultaneous measurement of the absolute distances of multiple targets; and designs and builds an integrated measurement instrument, which can realize high-precision real-time dynamic measurement of the multi-degree-of-freedom of the cooperative target. The technology has the characteristics of high stability, fast measurement speed and easy integration, and can meet the demand of high-precision pose measurement in the field of aerospace.

[0057] The model of each device in the embodiment of the application is not limited unless otherwise specified, and any device that can complete the above functions can be used.

[0058] Those skilled in the art can understand that the drawings are only schematic diagrams of a preferred embodiment, and the above embodiment numbers are only for description, not representing the advantages and disadvantages of the embodiments.

[0059] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A hardware real-time decoupling multi-degree-of-freedom measurement system based on double femtosecond laser, characterized in that: The system comprises: The measuring laser emitted by the measuring light source enters the beam splitter after passing through the circulator and is divided into three beams of measuring laser light. The three beams of measuring laser light pass through the first collimator, the second collimator and the third collimator respectively through the optical fiber and are incident on the first corner reflector, the second corner reflector and the third corner reflector on the cooperative target; After being reflected by the cooperative target, the three laser beams return to the beam splitter along their original path. After beam combination is completed in the beam splitter, they return to the circulator along their original path. The laser beams carrying the three-way measurement information will enter the cross-correlation module, and asynchronous optical sampling will be performed based on the local oscillator laser generated by the local oscillator light source. The measuring laser will be stretched in the time domain to obtain the period of the stretched laser signal. The stretched laser signal enters the photoelectric conversion module and is converted into an analog signal, which is processed by the signal processing module and converted into three-way ranging values. The degrees of freedom are calculated based on the ranging values.

2. The hardware real-time decoupling multi-degree-of-freedom measurement system based on double femtosecond laser according to claim 1 is characterized in that: The period of the broadened laser signal is: Where Δf r is the repetition frequency difference between the local oscillator light source and the measurement light source.

3. The hardware real-time decoupling multi-degree-of-freedom measurement system based on double femtosecond laser according to claim 1 is characterized in that: The system calculates the distance information corresponding to the three targets, and then obtains the multiple degrees of freedom of the targets.

4. The hardware real-time decoupling multi-degree-of-freedom measurement system based on double femtosecond laser according to claim 1, characterized in that: The signal processing module uses a dual-core CPU, CPU0 is responsible for reading the pre-processed data to calculate the distance value and multiple degrees of freedom, and CPU1 is responsible for controlling the three serial port IPs in the FPGA to communicate with the laser driver and temperature control module via serial ports.

5. The hardware real-time decoupling multi-degree-of-freedom measurement system based on double femtosecond laser according to claim 1, characterized in that: The signal processing module is: After asynchronous optical sampling and stretching, the laser pulse is converted into an electrical signal by an avalanche diode. After the reference pulse and the measurement pulse are A / D converted, they are pre-processed by two IP cores respectively. The pre-processing part needs to filter out signal noise based on dynamic thresholds, retain valid sampling points on the stretched pulse for peak fitting, and count the sampling intervals. When the sampled signal is greater than the set threshold, the preprocessing IP retains the sampled value; when the sampled signal is less than the threshold, the sent data is set to 0 to filter out all noise.

6. The hardware real-time decoupling multi-degree-of-freedom measurement system based on double femtosecond laser according to claim 5, characterized in that: The preprocessing IP sends data to the FIFO and writes it to the shared memory via DMA. The data sent by the measurement signal preprocessing IP is as follows: After detecting the rising edge of the reference signal valid flag, prepare to send data. If the measurement signal is valid, send the pulse interval clock count, and then send the valid sampling point. Until the measurement signal is no longer valid, send the stop bit to complete the transmission of a complete frame of data. Each time a group of data is sent, the sending count flag is increased by 1. When the sending count flag is equal to the measurement target number, continue to wait for the rising edge of the reference signal. Otherwise, continue to wait for the measurement signal to reach the threshold before sending the sampling time interval and sampling point.

7. The hardware real-time decoupling multi-degree-of-freedom measurement system based on double femtosecond laser according to claim 5, characterized in that: After completing the complete sampling of a pulse, the preprocessing IP transfers the data to the FIFO, which is then moved to the shared memory by DMA. The ARM core software reads the data in the shared memory and performs Gaussian fitting to obtain the peak position and calculate the actual distance.

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